Definitley let the process happen on its own. Clams will move if they do not like the area they are in. If you want to give it some encouragement to attach there, you can chip out an indentation in the rock or make a pvc bowl that you rest he clam in that can be pulled apart when the clam has attached. Get the appropriate sized pipe, gut it off to 2" tall max, then slice it in half and bind the halves together with a platic hose clamp or zip ties. Lay the clam in it and after the clam attaches to the rocks cut the zip strip and pull the halves away from the clam.