I’m leaning towards just being unhappy there. I’ve seen similar in my pink octospawn. But keep an eye on it if it gets worse.
With bjd, the flesh will recede and go brown. It will close up a lot, and when it dies, all the flesh becomes brown jelly.
Ciprofloxacin is known to treat it, chemiclean also has some success. One a coral erupts into brown jelly, it’s generally too late.