doesn't look like fungus. Fungus is white and fluffy and looks like a foreign object as opposed to part of the skin. This looks like an injury to me, the white stuff appears to be just peeling of the slime coat/skin from the injured site. keep an eye on it but it should heal up just fine. to eliminate stress and reduce the risk of infection you definitely want to do a pretty large water change to get those nitrates down. is there any tank mates or do you think he may have managed to get it stuck somewhere? he may have just gotten spooked and thrashed around too much and ripped it, sometimes that happens.