clam question

I found this guy moving along on the top of a rock opening down to rock is it realy a clam or something else it was moving towards a snail. It is in my QT with a new true percula I can't see it harming him. A few of my herits have gone missing out of their shells could this guy done it? If this is a clam i didn't know that they could up root them self and move once they astablished a ancor.
This is the best pic I could get with 10x macro lens it is about the size of a puffed wheat grain 0.5 inch long.

removed pic's.

Thank you,
Bill