Anyone know what this is?

[Borderjumper]

It's a chiton and it won't bother anything. Those Pyramid Snails on the other hand will eat your clam and kill it.

[Myka]

Originally Posted by Borderjumper

It's a chiton and it won't bother anything. Those Pyramid Snails on the other hand will eat your clam and kill it.

Agreed. The Pyramidellid snails will also attack larger snails, particularly Turbos and Astraea.

Here's some help: http://reefkeeping.com/issues/2008-02/nftt/index.php