Don't pull your clam off the rock. That can do damage that will ultimately lead to it's death.
Take the clam and rock out and use a sharp knife, or scissors, to cut the threads attaching it to the rock as close to the rock as you can get. If you canââ"šÂ¬Ã¢"žÂ¢t see the threads, then gently pull the clam away from the rock until you can.
Steve