may have been rubbing it on the glass or also possibly lymphocystis...side-view shots may also help. for now, i would just increase water temp to 82F and increase water change volume and frequency to 50% 2x a week and see how conditions improve. 1tbsp of salt per 20g water also wouldn't hurt...
...basically standard procedures for injuries/gar sickness. hopefully things improve from there onward, but keep us posted--
--solomon
but hey dont kiss him. looks like common case of herps . is there a dirty fish in ur tank. lol hope things go ok, good luck. i got a florida with that once on one side of mouth.an it went away in a few weeks time. never knew what it was or how it happened