Stripped thing I found, please help ID

[rocketlily]

I was just changing powerheads in my tank and looked into the back corner as something looked a little odd. This is an area of the tank you cannot see without a great deal of effort and I saw this stripped thing on top of one of the rocks. Sorry this pic is a little fuzzy, but it's very hard to take a picture of the back corner. I've never seen this before and I though I cooked my rocks well enough that I wouldn't see anything new again. Anyone have any ideas as to what it might be. It doesn't appear to have a hard shell.