There are hundreds of different type of Asternia, and yes some do eat zoanthids ( as evolved posted), this I know first hand. Some eat coralline, some other algae, some sps. Identification of the specific type is hard, colors are always variable as are numbers of arms, neither characteristic is definitive enough to make an accurate ID. Basically if you see them eating something that is good enough. Personally, I really like them as a member of my aquarium as long as they are not eating any corals.