|Maxima and Crocea Clams are "boring" clams, meaning they anchor themselves into position by boring into something stable. They prefer to be seated within a hard substrate such as live rock. The reason for clams to move about the aquarium, as yours is doing, is to try to find a suitable location to seat themselves.
It is a good practice to place any clam in the substrate until it becomes accustomed to its new environment. Bury a small rock that is about the same length as the clam just below the surface of the sand. Place the clam directly on this rock, giving it a place to anchor itself. This rock will also protect the clam's delicate byssus opening from possible predators. To provide a sense of security for the boring clams, surround the clam with several small rocks, but be sure not to inhibit the clam from opening fully.
Within a few days to a week's time, the clam should fasten itself to its new location using its byssal filaments. At this time, the clam should be acclimated to the new lighting, and it can be moved, along with the rock on which it was anchored, to a desired location.