No idea when or where it would have come from. Nothing new in tank for a few weeks. 1 close up and 1 further back for size perspective. if it really is a snail then I'm shocked the red coris wrasse hasn't gobbled it up.
Why do you think it has no shell? Looks like it does to me. Possibly a nerite species. Wrasses, in my experience, would rather attack snails that have fallen down instead of doing the work themselves.