hard to tell from shell but either a pond or bladder snail, looks kinda like a bladder snail but not possitive. Probably came from some plants you put in there. Eggs being stuck to plant leaves or stems if not treated or quarantined develop and you get snails.
They wont hurt anything unless you just dont want snails.
No it will not, unless they are dead, but will mostly go after algae and diatoms. You can use a turkey baster to suck him out if you want, or just smush him on the glass and your fish will do the rest. buy any driftwood or rocks that were in the tanks at the LFS?
Could still be from your plants and just been dormant for a while. The eggs just don't hatch immediately. That's the only place I think it could come from, but where there's one hitch hiker there will be more and they are quite prolific at producing babies.