My best guess from the pic is a bacterial infection but I'm not great with pics. You need a QT/HT to treat disease/parasite outbreaks. He may respond to a broad spectrum antibiotic, like Furan II.
Are these fish in your 28 gal tank? If so, its way too small for any tang, much less two larger tangs from the same genus. You have a small tank and should be keeping small fish. I wouldn't keep a sailfin in anything smaller than a 180+ gal. tank. The yellow will grow and need about 100 gal tank. These two fish generally hate each other and there is no way the two can be kept any lenght of time in such a small tank. Research the adult size of your fish, their growth surprises many folks. Just a pair of clowns and the coral beauty should work out, any more fish than that will ,lead to inevitable problems. Here's a good source of fish info, great source for fish too. (Liveaquaria.com.) If a dealer is aware of the fish you're putting in a 28 gal tank; I sure would find a new source.
Sorry for the negative comments; but they're needed. Stocking a tank like this will ruin this great hobby for you in a very short time.
