Looks like a slug with a small shell on its back but not one it can go in Just one that looks like it protects its back. By the way that’s not it shell that it’s on that’s just an empty hermit shell that I’m adding.
Also +1 on stomatella (wish you could buy them somewhere, they are maybe the best member of a CuC you can have). Keyhole limpets don’t have a shell on the outside of their body, it’s covered by their mantle and there’s a little ‘keyhole’ that you can see their shell through, but their shell is under their ‘skin’.