Brittany,
This looks like hlle. Hippo tangs are very susceptible to it and sometimes never overcome it. You also may have residual skin problems from when it was young as you note.
Hippo are not herbivores and they eat a lot of meaty foods. Continue trying other greens and seaweed but that likely is not your problem if the fish is eating Formula Two flakes each day. I also use Omega Veggie flakes. I would feed this fish better sources of meaty foods, including PE mysis, Hikari mysis (some fish eat one but not the other), chopped scallop, shrimp salmon and clam fresh from your grocery store, and NLS pellets. Also soak foods in Selcon.
I would remove the carbon for a couple of months. It isn't breaking ammonia down, although it does remove some other dissolved organic from the water. Your rock and protein skimmer handle the ammonia.
I think the problem is part diet and part stress. The fish sometimes being hyper may indicate the tank is too small. Is the rockwork open so that there is a lot of swimming room in the tank?
It might be a good idea to increase your water changes. How often do you do them and how much?
Are your nitrates zero? How do you test?
Your tank's temperature is on the high side. Not really a problem, but perhaps contributing to what is going on with the fish.
Get the fish high quality meaty foods, Selcon, no carbon, and lowered stress and the problem should clear up.
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