It looks like a saprolgenia fungus to me, it spreads quickly and can kill quickly, here is a pic of it on koi.
Ther treatment once established is harsh, first net the fish and swab the area with Hydrogen peroxide and place it in a bucket of well areated water that contains 3/4 pound of salt per 5 gallons water, keep it in there for about 10 minutes, watch it the whole time to make sure it doesn't jump out or go into shock.
Then place in tank, preferably quarantine, and treat the water with a malachite green-formalin mix like Forma-Green, Maroxy is also supposed to work.
The cause of the infection is a fungus but it can be brought on by scrapes and bruises, polluttion, sudden chill, or other things that cause major stress. Treat it soon.
If you are not doing it already I suggest you keep at least 1 tablespoon salt per 5 gallons in the tank water.
