Why are my green star polyp turning whitish from the outer side?

[Daylilly]

They have not opened up either for a long time. I've been giving the tank some DT thought they would open up....

Are they dieing?????
I heard it's hard to get rid of them

 

[joy13]

Flow might not be strong enough I have noticed mine open a lot more in a strong flow. What is your SG? Another thing might be ammonia, or nitrates.