I had a common red hermit crab (probably 3 inches) in my old tank that would attack any algae, especially macros, that i put in. I don't know if it would work for you, but they're cheap (i got mine for like $6) and they are easy to remove when the jobs done.
That being said, he also ate a few hairy mushrooms before i caught him in the act. Now he's in another tank by himself. So, be careful if you have other things in there he may have a taste for.