It's a little hard to tell in the pic, but is it fuzzy looking? If it is I'd say it could be a fungus, for which you'll need a medication. It might also be an ulcer, but they tend to be all over. Does the spot stay the same size or grow over time? Have they been spreading to other body parts? Is it waxy and yellow looking? Are there any other noticable symptoms that you have seen? Have you tested your water perameters? Ammonia, nitrites, and nitrates? Sometimes things can come in with plants, next time some are added make sure to quarentine them first.