Like all lobsters, they're scavengers, so if they can catch it, they'll eat it. They're pretty cool, although they only come out at night and reside in crevices in the rocks during the day. But definitely shouldn't be kept with other crustaceans (aside from hermits) or small fish that hang out at the bottom, like gobies and blennies. As for sessile invertebrates, you never know. If they're hungry enough, they'll eat whatever is around.
Ten gallons seems alright, if it's just him and some hermits/snails.