i wouldnt do the magic earser...my sister got a chemical burn with one of those....i would NEVER trust one of those on my snail for fear of shell erosion!!!
How about some algea eating shrimp? My algea eatign shrimp cleand the algea off my snails. There wasn't a lot there, but they kept the snail algea free.
I don't think the magic erasers have any chemicals on them from the factory. I'd imagine that the one your sister used probably came into contact with chemicals after purchasing it.