Coralline algae forms hard plates of calcium carbonate that are too hard for snails to get off -- if YOU can't scrub it off, there's no way a snail can get it off with it's soft mouth. Coralline is "good" algae in that takes up nutrients, inhibiting the growth of some of the uglier macro algae (hair algae, etc.). For this reason, a lot of people just let it grow on the back glass. If you really don't like it, you can scrape it with a razor blade or something similar. I've never heard of fish eating loose coralline, but if they do, I doubt that it would hurt them.
FWIW,
Mariner