I am no expert, but it does not look like any disease I have seen. Looks like abrasions marks to me. Also, possible bite marks from other fish (have you observed any aggression?), but I do not think so. Monitor his behavior and make sure he is behaiving normally and particularly eating well. If so, I think you have no issue.
I would also improve his diet. What you are feeding at this point is barely adequate. Add some mysis, minced table shrimp, and other meats to his diet (brine is very low in nutritional value) and nori/macro algae from refugium, if you have one, to his diet. I would also purchase some new life spectrum pellets to add into the mix which this fish in particular absoluely loves and is especially nutritious.