Usually if there is one visible there are a dozen that aren't. Whether it is harmful is a matter of opinion, and species. In a planted tank certain types of pond snails, generally round and dark coloured will cause mayhem. Another very common thing you'll get are Malaysian Trumpet Snails, these are longer and pointed, like an ice cream cone. These tend to bury in the substrate and can be helpful keeping it open, they don't damage plants as a rule.
There are loads of types really. I hate them all!