It could be the beginning of ich. Hippo tangs are very susceptible to disease, and unless you quarantined and treated the fish before adding it to your tank, it is probably carrying the disease. Most fish can have ich and never get sick from it or show symptoms, but hippos are not one of those fish. They tend to get stressed and get ich very easily. Do you have any other fish in the tank? If you want to catch the fish and treat it for ich, you will have to treat every fish in your tank, as they have all been exposed to it. If you only treat one fish, as soon as you put it back with the others, it will catch it again. Have you seen any white spots on the fish?
Is the fish still eating healthy? If it is, you can also try soaking its food in garlic extract and vitamins prior to eating. Lots of times, fish can fight it off on their own as long as they continue eating. But if that happens, ich will still always be in your tank.