What fish/Inverts do you have? Something broke through the skeleton. That's the inside of the coral skeleton. I would super glue over it and remove the problem animal to a new tank.
Yeah that’s the only thing I could think of. The head is about to come off as well. The only 2 candidates are a coral beauty and a fire shrimp which I doubt. I never noticed it because it’s always a very happy hammer and saw it last night when he retracts. This pick is from a few hours ago.