It's gonna be hard to completely remove them from the snail without harming it. I'm not sure how they reproduce, but I would treat them as Aiptasia, so don't cut them, that might make them spread. I would never suggest this, but for the sake of your tank in this situation, I'd say toss the snail... crush it with a brick outside or something, make it quick... As for stinging you, I doubt it. Good luck.