as long as the clam is able to extend fully most of the time, isn't trying to move out of the way, and the mantle isn't always folded over, i wouldn't worry about it.
the way i figure it a lot of the tridacna species inhabit places where currents and waves will occasionally surge with some decent force, so what you've described should be well within their normal environmental parameters.
for my part, if i put a clam somewhere, and it stays there, then i assume they're happy enough.