If it is something that is not from a scrape or other injury, it is a water quality issue. Datnoids need more cleaner waters than most other freshwater fishes. If it is an infection caused by lack of water change, it may fix itself if you continue to keep up with the water quality. That infection looks like it maybe on the lateral line of the fish. If that is the case, it is bad news. It's hard to control that one. Good Luck.