They wouldn't bother an anemone. They are pretty inactive unless they are eating, which they do in one huge gulp. They do not "pick" at things like many fish (angels, triggers) do, that is why sessile inverts are not at risk. They may eat a crab or snail occassionally (it seems like they may try it at least once, but I've not heard a lot of firsthand accounts of it happening), but probably not often enough to break the bank. Just keep it well fed if you want to keep collateral damage to a minimum.