Coloration of conch is what it looks like. Might be a rock though?, LOL.
Edit: You might want to keep an eye on your blenny too. Those little things are just head smashers against stuff when it comes to eating algae. You might have seen this. It could stress out a new clam if they come in with new algae (or even just algae from the tank) and the lawnmower starts rockin it's noggin against their shell.