Astrea snails, Astrea americanus, also called star snails, turbo snails, and algae eater snails, are one of the main stays for algae control in marine aquaria.  They are about the size of an average grape, and have a voracious appetite for algae.  They will eat just about any thin film, encrusting algae, but will also eat hair algae and other nuisance algae.  They don't seem to eat coralline algae, and usually don't bother caulerpas, most calcareous algae, and most fleshy macro algae.  They may forage on some of the larger algae, but unless they are really hungry they don't tend to eat them.  They don't generally eat bubble algae; so if you're having a problem with it, get some green or red mithrax crabs.