A circular growth like that, around a central point is often bacterial (that's how colonies of bacteria grow). Trouble is, what caused the initial lesion? Do you have another tang or foxface in the tank? A spine puncture is a common way this gets started.
The next question is to treat or not. There really is no effective treatment you can do in a DT, so the question sort of boils down to - leave it be and see if it heals on its own, or move the fish to a treatment tank (something a tang like this is not going to enjoy at all).
Jay