If that critter has a kind of undersized saddle-like shell on his back, that's a stomatella snail, a very good guy. But if he's all soft-bodied, that's either a flatworm or a sea slug, not a good thing. Wait for more opinions, just isolate him in a dish, and if that's a stomatella (I dimly recall they have antennae, which flatworms and slugs don't) put him back in and hope he breeds.