In my opinion, no planted tank is complete without snails. They will show up unbidden, and they mean no harm. Many are beneficial by eating the dead plant material and cleaning up uneaten food and algae. Looks like you have the ever so common bladder snail. I have them in my tanks too, and they make lots of babies really quickly. Keeping their numbers in check is all you can do at this point. Just make sure you aren't over feeding your fish and they shouldn't get too out of control. You can always add some assassin snails but they will also breed.