What breed of clam?
The big issue I see is shading. If your clam is a slower growing species like a Maxima or a Crocea, the frogspawn could end up shading the clam too much. The only other issue I see is the tentacles of the frogspawn irritating the clam's mantle. Not from stinging, but from intermittent contact which will cause the clam to retract its mantle and eventually may cause it to stop opening as much which can lead to a host of problems most of which end in clam death.
However, as pointed out, clams are capable of moving amazingly well, but that can result in the clam adhering to a spot you don't want.