If a clam is attaching to something small like that let it be. Just move the clam and that cradle if needed.
If you really want to remove the cradle, there is no need, you need a sharp knife of a pair of scissors. Tilt the clam to one side cut the threads you see, then tilt the clam on the other side and cut what you see. Keep on doing this until you remove the rock. Be careful just cut the threads not the byssal gland.
it is easy as the threads are cut the clam will loose it’s grip and you can tilt it further each time without putting a lot of force.