I looks to me like a growth and not a separate organism. A separate organism would probably only be attached to the fish in one small spot. Lots of fish get tumors easily and on appendages they will live for years without doing anything. But they do look unsightly.
If you want to do something or feel you must do something, here are some possibilities:
1. dip the fish in water that would be used in a marine tank. Dip for 30 seconds or remove to fresh water if the fish quits struggling, he will obviously struggle since he will be buoyant in sea water. That will kill any organism that is the thing or causing it. And probably this will do nothing in this case.
2. net the fish, wet a towel, place the fish in the wet towel and cover the fish with wet towel so only the tail shows. Use a razor blade or small scissors or toe nail clipper to remove the growth.
then do the salt dip to cauterize the wound and put the fish back in the tank.