If that's not a fungus, it's a nasty form of HITH. If untreated, his head will explode with 'cottage cheesy' white stuff and he will die soon afterwards. I've dealt with this in a few past fish (a fh, a GT, and a pacu). In the case of the fh, I treated with AP med to treat 'hexamitia'. The problem went into remission for months only to strike back with a vengeance. It's unsightly; therefore, once there is significant tissue damage and the fish stops eating, I'd euthanize.
POST THIS THREAD IN THE DISEASE FORUM ASAP!
GOOD LUCK!