looks like ich ive had it once i had good luck with feeding with garlic everyday soak the food in garlic extreme an hour before hope it works for you also..
How old is your tank? I only had ich problems in my first 2-3 years. Hippo tangs are ich magnets...I can't think of any other fish more susceptible to it. When you can introduce a hippo without having it get ich, you know its not a problem in your system anymore.
i have a big tomato clown that im moving to a different tank but my tank been set up for over 2 years now. i got the tang when it was about 1 inch. im gonna try the garlic extreme and see how that goes and def getting that clown out asap