it is hard to say. I agree with the sentiment if it is not bothering the severum and really not changing then let it be. I imagine it might be a scar at this point. At this point I would really consider just letting it be. If you really want to, you can pull him out of the water and try to scrub it a dry cotton ball in hope to roughen up the skin a little bit. Follow this with apply dilute betadine once daily for 14 days or so. This might encourage it to heal in. That being said the stress you are going to put him through probably isn't worth it. The third option is to take him to an aquatic vet. This ways skin scrapes, cultures and even biopsy can be done if needed.