well first is that what you have it also could be a calsium build up. if it is the sponge it is harmless but it is just growing in the wrong spot. the best way to get rid of sponge is to get a sponge eater but if that is out of the question than get some additives that get rid of sponge. if it is that calsium build up idk what to do.
Might be a digestive hemorrhage from damage. Keep it in good current and make sure nothing is picking on it. If that is what it is, it should heal on its own.