I agree with Krish, if your tank is large enough, you might consider a Tang. Keep in mind, they're a lil' more intolerant of Ich though. Also keep in mind that once they've devoured your algae, you'll have to supplement their diet. Might look into a Lawnmower Blenny for the hair algae. I don't know of any crabs or snails that are going to really help out with Calurpa though. I got rid of mine by picking it out by hand whenever I saw any cropping up. After a few trips into the tank by hand, I've not seen anymore in a very long time.