I've had my Derasa for a few weeks now, it still hasnt attached itself to the sand or the bottom of my tank. It seems to rotate around a bit, and I've moved him to a few different places on the sand bed, trying to get it in a spot where he is happy (and not moving around) and its easy to see his colors, its an awesome looking clam.
But I've read that of the Tridancid clams, T Derasa takes the longest to attach, and its attachement is no where near as strong as those of the other Tridancid species. Mine still reacts to changes in the lights or when fish buzz right over him, so I assume he is healthy.
Does your clam still adjust to changes in the light? If you create a shadow over the clam, do you see a reflex from it? The owner of the LFS where I got mine showed me this, and said if a clam ever does not react to the shadow - its not healthy