Oh dear! Maybe there were snail eggs in the moss. They are tiny little things! I use "Sluggo" pellets for the big snails we get here. But maybe you can just wash your plant down real good and repot. Better check all your plants really good, too. Some people just put a thin slice of apple right on the surface of the media in the evening and apparently it will draw the snails to it and then you can just discard them. Good luck!