Please help st thomas coral

[T33BLACK]

So I believe this is a st thomas and I've had it about a year.The big one in the middle was the original and all the small ones around have been popping up over time .It's been looking bad for a few weeks now but everything else in the tank is fine. Water parameters all seem to test ok. I did a healthy water change last week to see if that helped but it didn't do anything. Being that all the other corals are fine I'm at a loss. Any ideas what it's doing or if that's bad?? would be appreciated

 

[Timfish]

Well, corals can have not only species specific responses to changes but also genotype and most liekly variety specific responses as well and certainly specimen specific responses. So more information is needed. What are the the water and lighting parameters? What changes have been made in the last few months?