It is most likely a bladder snail. They most often come with plants, mosses, etc. They also often arrive as eggs, and it seems as though they appear from nowwhere, when the eggs hatch. They are not harmful, but they can multiply quickly and many do not like them around in any great numbers.
They sometimes will make small holes in the odd plant leaf, but for the most part they are like garbage men, eating dead plant matter, old fish food and whatever else they finds around, including algae. So they can be reasonably good cleanup crew. You just want keep an eye on the numbers, because they are very successful at making more of themselves fairly quickly. They are hermaphrodites so they do not need mates to lay viable eggs.
They will lay eggs, tiny irregular blobs of clear jelly with tinier dots in the gel, on plants, rocks, glass, etc. If you see one of these blobs and scrape it off, it prevents that lot from hatching. You can even remove them from plant leaves, by gently squeezing the leaf from both sides. Most leaves survive with little damage.
Really your choice to keep it or toss it. I don't mind a few of them around, but if I see a lot of little ones, I know eggs have hatched recently and will remove most of them just to keep numbers down.
People who keep puffers actually love these guys, and breed them, to feed the puffers. Some loaches also love to eat snails, and their keepers also let snails reproduce to feed the loaches.
Personally, I have yet to meet a snail that was so useless I would just get rid of it without a thought, but you do have to keep populations under control. Some snails, like the giant size Apple snail, will eat plants, so they are not welcome in planted tanks, but are fine in those without plants. It's more of a personal choice, to my mind.
