RegalMan, clams lie on their sides for a variety of different resons. Like people, every clam is different, but here are some possibilities:
1) He was previously kept or has been acclimated to laying on his side. In which case you can try to acclimate him facing upwards - but this is sometimes a losing battle and can cause more stress than is necessary. I have a 3" blue maxima that is like this.
2) Something is ****ing him off in your tank. If he is under new, higher intensity light, he'll curse you and lay on his side. You say he likes swimming in the sand? Croceas in general don't like sand because it irritates their byssal opening. They like rocks, so, if your clam isn't already attached to something, put a small, flat piece of coral rock in the desired location and position him on top of it. It might take a day or two to get used to, but if he likes what he feels, he'll secrete threads in a few days to secure himself.
In any case, if you don't mind a little localized bleaching where there are shadows, your clam won't care much. If you look in the literature you will find that croceas are found in all sorts of weird positions in the wild.
