I think it's the black beard algae. No i plucked all the sanils out. I can't stand them. Although maybe your referring to a different type of snail. Is it diffrent to the ones that get in there from plants?
All snails will eat algae, but like fish they are picky as to which species they will eat, and generally this will never really solve "problem" algae issues. The common algae that naturally forms part of the bio-film on all submersed surfaces is rarely even seen by us, but readily eaten by snails and some grazing fish (otos, Bristlenose, Farlowella, mollies, rift lake mbuna, etc).
A comment or two on snails...these small snails that may hitch a ride on plants or wood are your best friends in any aquarium because they eat organics (fish excrement, uneaten food, dying plant matter, algae) which breaks it down faster for the various bacteria to deal with. The organics will be there with or without the snails, and the snails add nothing to the bioload because they are only eating what is already in the bioload (unless one specifically feeds certain snails, not the issue here). So the snails are doing you a favour.
As to their numbers, they will exist at the level according to the available food. Even if you are careful not to overfeed the fish, the number of snails indicates just how much organics can accumulate in an aquarium, and this must be kept under control.