In addition to advise already offered, I think that information about your aquarium & your routine could be helpful in diagnosis. For example, what are your water parameters? What supplements are you using and how often? Water changes, how much and how often, CA reactor? Calkwasser? - et cetera.
Aside from water parameter issues, Lighting can be a major issue with Crocea clams. They are the most reliant of light of all the tridacna's as they are indigenous to very shallow water. If this clam has been put into the same circumstances (placement, feeding, light type, et cetera) as the other species you currently have, it isn't going to do well long term. Compare this to putting and Acro and a mushroom side by side in an aquarium and expecting they both to do well under the same conditions. Neither will die right away but one will eventually because of their radically different requirements. Many (myself included) believe that Crocea will only do well under metal halides. I've done best with them placed up high in the rock close to 175 or 250 watt M/H with a 10K-20K lamp. Others have reported that even 400 watt are the way to go.
Feeding Crocea can be tough as well, they feed only on the finest (smallest) of particle foods which can be a problem with some of the regularly sold filter foods. Home made whisked yeast or live phyto plankton are what some try feeding corcea.
A lot more information could help with trouble shooting this question.
Joel