Looks like post injury with hemorrhaging. has any other fish been chasing or bothering it?
Is fish eating ?
Maracyn in a seperate tank should help with this
He's been in there a year with no recent additions and I've never seen him get bullied. I was thinking about it some more and I think he may have taken a Diadema urchin spine to the face. That would be consistent with the symmetry and the dark brown/purple color. He seems fine so I'm just going to wait it out and monitor.