I would QT, and treat with antibiotics (i.e. Furan-2, Erythromycin, or any broad spectrum antibiotic).
IF this fish has Brook, you will see patches like this show up on other parts of the body. For that, you will need to administer formalin dips. Personally, I would administer formalin dips regardless (just to be safe). The antibiotic treatment is a good idea either way as well, to guard against a secondary infection.