Removing a clam from a rock????

[Delphinus]

Depends on how easily you can get under it. It's very risky. In theory, if you can slice it right at the rock you can do it, but you run the risk of pulling the byssal gland out or nicking it and it really doesn't take much to be game over with clams.

If you can reposition the rock such that it doesn't want to stay on it anymore, it can let go on its own within a day or so.

How certain are you of the flatworms? Can a freshwater dip take care of the flatworms? The clam can survive for longer than you think in a FW dip... this is the recommended treatment for clams with pinched mantle for example which is, IIRC, usually a protozoan to blame for.