IrishStout,
A hippo tang won't eat green hair algea. The green Seaweed Selects mentioned earlier will be a favorite though. The hippo I had ate just about anything I'd feed....loved brine shrimp, krill, clam. Make sure you provide the green seaweed regularly....they need that no matter what else you're feeding.
I have to tell you though that I think it's a mistake to put a hippo in a 55 gallon tank. He will be too big in no time. I had mine in a 45 and he was happy about 6 months until he got to be 4-5" long. He soon died of ick. I now know the ick was a result of the stress of having him in an area that was too small. Most of the experienced reefers on this board agree that a tang shouldn't be kept in anything smaller than a 75.
Jim