From what I can see it seems to likely be a skin bacterial infection. Some are benign (not harmful), whereas other eat the skin right off the fish until it dies. The rest are inbetween.
Is the fish eating? If 'Yes' feed the best foods that are sea foods that the fish should be eating and monitor it closely.
If 'No' then move the fish to a QT and treat with an antibiotic and work at getting the fish to eat. Also, treat like this if you see it getting worse even if the fish is eating. I'd suggest any topical (surface) antibiotic will do. While you're monitoring the fish, you might go ahead and obtain some and have it ready.