It looks like black spots disease... It can be found in wild caught fish especially piranha, silver dollars, and other Myleus species. It's a parasite that requires birds as an initial host. Birds eat an infected fish, and defecate into the water, and then the parasite searches for a fish host. (I believe there's another species that requires snails as a secondary host.) There's really not much you can do (or should do). Just leave it alone and let it go through it's life cycle. There are meds such as Clout that claim to treat for it, but I wouldn't stress your fish with unnecessary meds.