IME, BTA’s don’t like a lot of flow and like to be fed. Also, if you have recently changed anything on the system, they may move. They do not like algae growth at all... I had one BTA, that split into two more (before my tank crashed). I noticed when I came home to the crash, they were all across the top of the tank. They had not moved in a year of being in the same spots; only when I had the algae outbreak. Would bubble up whenever I fed them pieces of silversides.
Are there corals that maybe sting this particular one as well?
Has anything at all changed?
Is it starting to shrivel up more than usual?
Are you feeding anything more than mysis shrimp?