Sometimes croceas just act weird for the first week or so that they're in a new tank. As long as the mantle is extending a little, and its reflexes are still intact (i.e. it shrinks back from time to time), then it should still be okay.
The reflexes you're seeing aren't caused by water flow, but in fact a reaction to a shadow falling over the clam's mantle (like a fish swimming overhead). This is their standard reaction, and it's a good sign.
If it stays this way for an extended period of time (like a week), then I'd start to get worried... chances are it's just a bit stressed out from the change, and on top of that you keep changing things hoping that will 'fix' the problem (which actually compounds it). I think if you keep things stable for a few days, it'll stop being timid and will (hopefully), start acting normal.
Keep us posted.