With good reason... I've never seen that before to include it in the guide! Is there a snail's food under there? Have you added anything in particular lately. If you touch it, is it fleshy?
It has a hard shell a small foot comes out attaches and almost like it's dragging its self around I'm thinking clam or something I also didn't add anything just end up finding things when I turn the lights on in the middle of the night