Tons. They won't harm the anemone (they'll both settle into a pattern). They're totally peaceful and filter feeders (so you'll see them out with their "nets" whenever they're trying to catch something). Awesome to have in your reef tank!
Totally safe and they won’t actively harm anything they host, but they can irritate it. One of mine hosts a St Thomas mushroom that is only maybe twice the crabs size. I wouldn’t the say the mushroom is thrilled but doesn’t really hard it.