Looks like the result of an injury and not a disease. Maybe he bumped into some decoration or something. I would treat with epsom salt. Use a one time dose of 1 tablespoon per 5 gallons for a fish only system or use 1 tbspn per 10 gallons/reef system and repeat after 3-4 days if necessary. Add to your sump or pre dissolve before adding it.

You can get epsom salt at any HEB (just make sure it's 100% magnesium sulfate) for a few bucks. Magnesium sulfate is found in all salt mixes and is very safe to use. I have used it many times to treat popeye and other eye problems in fish only and reef systems with no problems. It usually does the trick, far better than antibiotics IME.