Not all snails but alot lot only need one to reproduce. There just like earthworms they are both female and male, so you may start off with one but end up with many. If you watch what kind of snail you get though it should be fine!
Though some types of apple snails eat plants, make sure you know what your getting. Also do you know your pH? Because if your water is acidic it will corrode your snails shell .
unless it's a mutent snails that eats pies, it should be fine living off of letuce, and the algae. they also like cucomber, watermelon and pretty mcuh all vegies.
what type of snail are you getting? if your getting just one i suggest an apple snail.
Ive no idea what sort of algae it is even from the exalent sites, kind of like slightly hairy green spot that covers areas of the tank, not in spots like green spot. From what others have told me, nerites also eat beared algae.