Clams are not found in sand in the wild. They are only found firmly anchored to reef rock, with such strength that it is often impossible to slip a dive knife blade behind the shell (take it from someone who knows). Several species of clams actually "grind" into the rock, so that by the time they are 2" or larger more than 50% of the clam is below the rock surface.
Clams CAN be kept in sand in a reef aquarium since there are no tropical storms or large predators. However, don't expect a clam to always be happy about it. They will probably try to find a firm substrate to anchor to, since that is the only way they can position themselves (relative to the sun, etc). In a deep sand bed there is no way for them to maneuver, so they will often fall over. Also, the only way clams can expel wastes, grit, etc, is by violently closing their shell halves. When this happens the clam can topple over, etc.
Is your clam falling over something to be worried about? Not really - just put him rightside up. However if you get tired of adjusting your clam, you might consider putting him on a bit of rock. If you want the "clam in the sand" look, consider burying the rock under a little snad.