First of all, awewome TMac. I have seen this many time on bigger fish when they slam into something...like a bruise. Never that large though. In a different circumstance, I saw a PBass that developed it on one side of its face. We read up on it in a cichlid book and they attributed to unexplained nerve/pygment damage that is typically rare. They further said this is obvious speculation and are not truly sure. The bass had it for about 5-6months and then it mysteriously went away, without treatment. Good Luck