Green Star Polyps Not Opening? (11 Reasons Why)

GSP corals are known to be a fast-growing and hardy coral that can handle less than ideal water conditions.

green star polyps not opening

They are a type of coral that you either love or hate. Many people don’t like them due to the fact that they grow so fast and can take up plenty of real estate in your tank.

I am a huge fan of them myself and keep them in my current reef tank. The great thing about them is if they are growing too fast for you, they are one of the easiest corals to frag.

So you can cut them back a bit and perhaps sell or trade them. 

Although they are a hardy and tough coral, there are times when they don’t open up as they should.

Most times these corals are vibrant and love to flow in the water current.

But that’s not always the case and there are reasons why they don’t open up as much as they can. 

In this article, I will show you the reasons why your Green Star Polyps are not opening. 

How do you know if Green Star Polyps are not open? 

GSP corals have neon green or pale green-colored grass-like tentacles that are extended and flowing when fully open.

When closed, they form a purple color mat and the tentacles will be retracted and out of sight. Only the purple mat can be seen.

My GSP coral showing the purple mat when polyps not open

Reasons why Green Star Polyps are not opening

  1. Water flow
  2. Lighting
  3. Placement
  4. Change in parameters/temperature
  5. Acclimating them to a new tank environment
  6. Critters crawling in them copepods, snails, shrimp
  7. Move to a new location in the tank
  8. Water changes/glass cleaning
  9. Fish nipping at algae around them
  10. Being fragged
  11. Lights off at night
green star polyps not opening
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#1. Water Flow

For most corals, the amount of water flow in your tank is a huge factor in determining how a coral reacts. The overall health of a coral is dependent on water flow.

Green Star polyps don’t require high water flow but they will need low to medium flow rates. If they don’t receive the proper amount of water flow, they will stay closed up and not open.

Too much flow or not enough flow can cause their tentacles to stay retracted. They love water movement that is either pulsing or variable flow rates.

Constant water flow that is too strong or pointed right on the coral will prevent them from opening.

#2. Lighting

Another reason why Green Star polyps won’t open is due to lighting issues. These corals prefer a low to moderate light. High lighting will cause stress and they will not open properly.

Make sure to position them in the tank where they receive low to moderate light. Some GSP corals will do well in shaded areas of the tank where the light is not directly on them.

GSP corals will also stay open if you use an actinic blue light at night. They look amazing under actinic lighting.

green star polyps care
One of my GSP frags under blue actinic lights

#3. Placement in the tank

Green Star polyps will be happy if you place them in the right area of the tank. It’s best to keep them in the lower to middle sections of the tank where they will be out of the intense lighting and water flow.

They love to be placed on top of the rocks. If you have a small frag of GSP, you can place the frag either on the rock or in a hole or crevice and they will naturally grow onto the rock. 

#4. Change in water parameters

Ideal water parameters for Green Star Polyps:

Salinity: 1.025

Temperature: 78-79F

Calcium: 400-450ppm

Alkalinity: 8-11 dkh

Magnesium: 1200-1350ppm

Phosphates: 0

Regardless if you are talking saltwater fish or corals, it’s important to maintain water parameters as stable as possible. Fluctuating parameters cause stress to livestock and it is easy to see when they are stressed. With green Star polyps, they will show this by not opening.

A change in water parameters can sometimes prevent polyps from opening fully.

This can be evident when you might get a change in salinity from evaporated water that doesn’t get replaced.

An auto top off system is great for keeping salinity levels consistent. The Tunze Osmolator is my top choice for auto top off system. Grab one on Amazon here.

If your GSP is staying like a purple mat and not opening at all, check your water parameters.

This includes water temperature. 

#5. Acclimating to a new tank 

If you are introducing a new Green Star polyp coral into your tank, it is common for them not to open up for a period of time. They need some time to adjust to their new environment.

Acclimating new corals is important. As they get used to their new home, they will start to open up slowly. 

#6. Critters crawling on them

It’s possible for certain invertebrates and other critters to crawl on GSP corals which can be stressful for them. It’s common for copepods to hang out in them as well.

Cleaner shrimp, hermit crabs, and many others like to crawl around GSP corals and this will cause them to not open up like they normally would. 

Check out the Green Star Polyps coral care guide here. 

#7. Move to a new location in the tank

Anytime you decide to move Green Star polyps to another location in the tank, they will close up pretty quickly.

As soon as you pick them up they will close up and become a purple mat until they are placed down and get comfortable again.

If you take them out of the water they will also close up fast.

#8. Water changes and tank maintenance

Whenever you are performing regular maintenance on the tank-like water changes and cleaning the glass, it’s possible for GSP corals to close up.

When you are draining water from the tank you might notice if the hose or any piece of equipment is near the coral, they will close up and not open. 

#9. Fish nipping around them

You may have certain fish that like to nip at algae on rocks which can cause some stress to Green Star polyp corals that are placed on rocks. Fish like Yellow Tangs, Angels, Butterfly, all nip at algae on rocks.

If you have a Mandarin Dragonet Goby, they will eat copepods that can hang out in GSP corals. This will prevent the coral from opening up properly. They will stay closed until the fish stop nipping at the rocks. 

#10. GSP being fragged

I mentioned earlier that Green Star polyp corals are one of the easiest corals to frag. The purple mat is easy to cut and place onto a frag plug, disk, or rock rubble.

When fragging these corals, they will not open up and their polyps will be retracted.

Coral fragging is very stressful and it’s expected for them to not open up during fragging. 

#11. Lights off at night

The last item on the list that will cause GSP corals to not open up is when the lights are off. If you are using moonlighting on your tank, they may stay open somewhat at night, but once the lights are off the coral will be fully closed.

Throughout the night while the lights are off, the Green Star polyps will remain closed until the lights are turned back on. Sometimes in the morning when there is some daylight in the room, GSP corals will start to open.

I notice that with my GSP all the time. They are usually open before I have a chance to turn the lights on. 

How long does it take for Green Star Polyps to open?

The time it takes for GSP corals to open will vary based on many factors. Water parameters, lighting, water flow, tank placement, all play a role.

But on average, it will take anywhere from 15 minutes to 1 hour for these corals to open. 

Green Star Polyps growth rate

GSP corals are a very fast-growing coral. Depending on your tank’s conditions the growth rate can vary. It doesn’t take long for these corals to cover any area of your tank.

They can grow on the glass and equipment in your tank. It’s common for people to place these on the back wall of the tank to let them grow out and form a GSP coral wall.

Green Star polyps can grow from a small frag to a full colony in about 1 year. Expect growth rates to be around 1-2” per month. 


Green Star polyps are one of the most common corals to have in a saltwater tank. They are hardy, have amazing color, and grow fast. If you have your water parameters stable with proper lighting and water flow, these corals should stay open most of the time.

Any one of the 11 reasons mentioned in this article can cause them to stay closed and not open up.