They will "swim" as melosu58 pointed out. They do that generally when they are not happy. I would recommend placing it on your rock structure since that is were they are naturally found. It would also benefit by getting more light as well this way- they require a LOT of light. MH
is the recommended/preferred source. Placing it on a small piece of LR
or a clam half-shell will give it something to attach it's byssal threads to, as well as allow you to move it if necessary. Once the byssal threads are attached the clam can't be removed.
FWIW I have mine on my sand bed, but it is attached to a clam half-shell that is buried. Be careful not to get sand in the inlet or outlet siphons. It can clog the gills of the clam.