Diet depends a bit on the species. The two species most commonly sold (and Aplysia species and Dolabella auricularia) will eat chaeto, soft macro, nori, and dried seaweeds of just aboutany type. Some Caulerpas give them a bit of a tough time because of the diameter of the stems. The only limiting factor is whether they can bite through something; that's really it for those two. Another related species I just encountered (similar to the Dolabella species but has blue dots and is a lot smaller) is a pickier eater and only wants really really soft seaweeds. If your sea hare is at risk of not finding enough food, best to stick some nori or other sea weed in for it to eat. It may eat the dried foods preferentially to some other algaes in the tank, but that's far better than it starving to death.