If it is a pond snail, bladder snail or ramshorn, it is likely to be reproducing fast. They are all hermphrodites that can lay eggs, and do, without a mate being needed. You may well see more if it laid eggs, suddenly a bunch of tiny ones will show up.
They are good clean up crew. But I usually pick out bladder snails I see, to keep their numbers down, as they can kind of take over if food is plentiful and conditions favour them. I don't think I've ever seen an actual pond snail in my tanks, but the small dark coloured bladder snails show up regularly.
I toss some of them into my critter culture tank where I am trying to grow sufficient numbers of ostracods, daphnia and other small life to use for fish food, as they seem to help keep the cultures going.