Check if it is attached. If so, you can "clip" the bysal's very carefully. As close to the rock as possible. If the foot is attached, I would leave it where it is. I think that is a "sign" that it is "happy" where it is. If you can pick up the rock it is on and move it where you want it, that is the best thing.
I have razor bladed one and moved it. the foot was not attached very well and was not a problem. Just be careful. Better to move the rock, or chisel then killing the clam.
Good luck...... I would just leave it. If it is not happy or having problems you will know and it will more than likely try to move on its own. Then you can put it anywhere you like.
Just my opinion, do what you feel comfortable with.
