Does it look like worm thing coming out, or a cancer? If it's small and worm like, it may be an internal parasite, and there are several treatements, depending on what kind of parasite it is. Cancers are harder--no good treatment exists.
If it were me, I would get the fish into a quarantine tank, treat with melafix, and wait. The parasite will likely come out entirely (don't want this in the main tank--could infect other fish), or the fish will die. Sorry--not a good prognosis.