The powerhead might work, but be very careful not to blast into the clam but rather at it's base. A flow pump like a Seio/Korlia/Tunze would be better. And make sure you can't risk blowing air into the clam.
Another possibility is to move it to a partially shaded area where it's aware of where the light is and that it's not completely in the light. That might cause it to move, but I'm betting it would take time.
The razor does work fine, though, if done carefully.