Usually that's after a long time of algae growth, because it won't happen like that overnight. Also, i've seen a lot of tanks that have GSP's covering the entire back glass since those corals spread like wildfire once they get accustomed to the tank, which usually takes very little time.
Also, the snails won't be able to eradicate ALL of the algae.. For something like that you would need a hefty amount of snails. They will "contribute" to the cleaning of your tank, whether it be algae, detritus, fish waste, etc.. But they won't do everything for you.
Snails, Hermit crabs, Shrimp, other crabs, are mostly all considered a CUC since their diet consists mainly of algae, waste, and detritus. Most of the inverts we can put in our aquariums mainly graze or rocks, and the sandbeds looking for detritus and algae. So yeah.. i would consider almost all the inverts we can keep in our tanks to be a CUC. Except for things like anemones, but they may be.. i'm not sure because i don't know much about those creatures.