I've found a mix of snails is good for cleaning up various algae, though none of mine really touch the nasty thick stuff. I've had great luck with astreas, ceriths, margaritas (though I lost my margarita due to not acclimating at all... acclimate them like fish, I've had much better luck with keeping them alive past 2 days).
I'm going to have to drop a plug for a brittle star here too. They get large, but they're so damn cool. I see new behavior from mine every day. Be sure to supplement its scavenger diet with small shrimp or pieces of shrimp if you go for one though, or any star for that matter, as a 20 is still considered a nano.