It would have to be an extremely quick dip. If the clam opens its mantle there is huge possibility for osmotic pressure changes (ie cells uptake the freshwater due to the osmotic pressure, causing rupture of cells) and could kill the clam. Can you not just either scrub the flatworms off with a toothbrush with the clam out of water or pick them off with tweezers or something??
Christy
PS all that osmotic pressure change stuff may possibly be BS
IMO I wouldn't risk it though.