ID this please?

[Instant Tang]

A friend of mine recently found a strange creature living on a piece of live rock...I don't know what it is and his wife is worried! LOL.

Can someone identify the strange valved/shelled creature just below the bubble coral in this photo?

It's shadow sensitive, when he gets close it closes up.

 

[mbbuna]

no way to id it for sure but it won't hurt anything