They are herbivores. Macro algae, blanched lettuces (romaine, green leaf, red leaf, collard greens for example), nori, herbivore preparations (frozen, pellet or flake) and they will eat any meaty foods you add to the tank.
Offering nori on a clip will keep them busy and reduce the risk of coral nipping.
Mine has always been my favorite fish once he settled down. Fairly peaceful fish. Don't get.stung by the fins as its worse than any bee sting you ever had.
Mine is a complete pig. He devours nori on a clip as well as anything I put in the tank (mysis, Rod's Food, Emerald Entree etc.) He also grazes on the rocks. They are not aggressive and are wonderful fish once they settle in. They can be skittish for the first few weeks, especially in quarantine. Highly underrated fish, IMO.