Algae can't harm them. The reason everyone tries to get rid of it is because it looks gross, not because it actually causes damage (unless, for example, you have live plants/corals that the algae might smother).
Turn the aquarium lights off for a few days (the frogs will be fine, they don't actually need light) and do some water changes, you'll get rid of it in no time
. I actually don't recommend buying random snails to combat algae problems--different species have different needs and they usually don't eat it all, anyways. What kind of snail did you get? If you're keeping it long-term, make sure you research the species
. They can be neat little guys!
As for the rocks, you can either:
-Scrub in boiling tap water
-Wash with a diluted bleach solution (wash very well and use dechlorinator afterwards)
Technically, copper will kill off algae, too (most products designed to get rid of algae contain it) but I would avoid it because there's a high chance you won't be able to remove all of it. This is bad in your case because frogs are sensitive to heavy metals (and it'd probably flat-out kill the snail).
Saltwater probably works, as Chinchi said, although I haven't personally tried it (although I have washed off saltwater rocks with freshwater, haha!).
By rocks I'm assuming you mean decorative rocks, by the way. If you're talking about your actual substrate (gravel), you can't do much besides wash it with tank water and vacuum it--using bleach or something could kill your beneficial bacteria.
Oh, and don't always assume algae growth is a bad thing. A little algae is actually useful--algae consume ammonia and nitrates, keeping things stable
. They give off oxygen, too, like a plant.