That's a bacterial infection...I think I would attack this in two ways...one assume that there's a parasite and it's breaking through the mucus layer. So I would shotgun the tank with something like Quick Cure. A scope would be nice as a quich scrape would ID any possible bug
The I would assume that the bacterial infection is a secondary invader...once the parasite is gone, the bacteria should be east to clear up with Furan or something like it.
G