I feed my Hippo tang Julian Sprungs (i think its called that) Purple Seaweed, as well as mysis, some flake, pellets, brine shrimp.
I vary what i feed them. Like for example, yesterday i gave them some flakes in the afternoon, then put some seaweed (i attach it to a small rock with a rubber band) and towards the end of the night i give them some mysis.
Not sure if there's anything else i should feed them.