Watch it or see a vet. I don’t think that medications are the answer. I would just see if it grows. Unfortunately, if it turns out to be melanoma, the chicken might start to suffer, and the best thing would be to euthanize it. Again, I am not a vet, and vet care is the best. A necropsy after death by the state vet would be the best way to get a definite diagnosis. Here are some organs with melanoma from the article above: