I can't speak to the symptoms, but having owned a crocea now for over a year, browning, just as with corals, is a sign of needing more light. I would check to ensure your lights are not in need of replacing and / or the clam should be closer to the top of the tank. Oh and yes, crocea's are found in rock work, not in sand as most LFS
's have them placed for sale. If you want to try anything, before going after radical checmical changes, I would place the clam up closer to the light. HOWEVER, know that it will attach and you will not be able to move it once it does attach. Much like an anemone, damaging its 'foot' could lead to death.
Best of luck!