If mama snail is leaving the water, she is probably looking for a place to give you more babies.

especially in a new tank, she will find just the right spot before she lays. If they're hanging out at the waterline a lot, it might be something in the water, your water stats are stable though right? The tank is cycled and all that good stuff? Although mine do rest near the top too. They seem to like it there because if they need to take in air, they don't have to move anything but their syphon. They can get lazy like that.
My name is Amy, and I am addicted to apple snails.