Is it a stomatella? Go to Melev's Reef - Visual Identification and go to the Snails section. You can see a picture of one there, they have a sort of half-shell on their backs (almost like a fingernail). I have tons in my tank, they are nocturnal and excellent cleaners.
I think thats it. The one I saw was more of a dark charcol grey color but the description sounds right. Cant believe I've never seen any before. There not exactly small. He was just crawling around in broad daylight, but so are all the other creatures that normally hide. I hope they have sense enough to go back to their normal behaveior when I put the fish back in. This is there chance to be friutful and multiply so maybe some will servive.