One Of My Live Rock Hitchhikers, What Do I Feed It?

[H2Con]

Here is a pic of one of the hitchhikers on the live rocks I got from a lfs. I don't know what kind it is, clam? Its about 3/4" wide in size. What should I be feeding it? Its been in my tank about 10 days and has stayed in the little space on the rock with only slight changes in position. Usually when I get close it closes up.

 

[SkiFletch]

At that size, phytoplankton/rotifers are probably the only foods you could "feed" it, even then it'd be a tossup as to whether it lives or not. Clams that small have low survivability rates in captivity, regardless of the species.