this has nothing to do with the heater most likely, I wouldn't worry about the heater at the moment ...make sure the parameters aren't off, and perhaps treat for bacterial infection, it looks like an infected wound of some sort.
Looks like some sort off lle that has become infected you should also start doing 15% water changes everyday and check for nitrates and ammonia what kind of filtration do you have? Whats ph? And what do you feed him?