Then it most likely wasn't kept in good conditions at the LFS or vendor, which is not uncommon. When buying clams, always look for a healthy growth rim on the shell, avoid all clams that don't have this unless you're willing or capable of providing the right conditions for the clam. My guess is the clam will move or release itself at some point, which will give you an opportunity to move it to better spot/position. There's a method to cut the byssal threads at the attachment point, but if you haven't done this before, it can be nerve racking and/or stressful for both you and the clam. Damaging the byssal organ is not something you want to do, as it will most likely lead to the clams death if the damage is severe. You might be able to put a piece of rock or rubble next to the attachment point in hopes that it moves to that piece, then you can safely move the clam.