That is some kind of mesostigmatid mite. Mesostigs are extremely diverse. Most are predators but some are parasites. Without slide mounting the specimen, I can't identify it further. My guess is that it's probably a predator and may be feeding on other microorganisms in the snail enclosure. If you want to get rid of them, the best control will be to remove the snail and completely clean out the enclosure they're reared in.