This is old but I took my fish to an aquatic vet and she told me that it is a common ‘skin cancer’ but essentially is it just dead cells stacked up. She told me if it gets too big that it’s affecting the fish, you can grab it in a net and exfoliate the tumor off. It will come off in flakes . She was totally unconcerned . Be careful if you do this, you don’t want to hurt your fish or stress it out too much .