From RC:
These clams require the most light of the Tridacna family. They occur in the wild mostly in very clear waters less than 15 feet in depth. (6 meters according to J.F.) The minimum recommended lighting requirement for a healthy Crocea is under a 150 watt metal halide of a reliable 14k bulb at a maximum depth of 20 inches, preferably less. To be on the safer side, a 10k bulb is recommended , and is closer to the "true" color of natural sunlight. I have seen some Croceas under 70 watts of halide, but these are normally placed high up in the tank. 70 watt bulbs are also considered by some as the least developed bulb, therefore being more inefficient in terms of their output compared to their 150 watt counter parts. 150 watts of (14k) metal halides would be the minimum ?safe? amount of lighting. They will do even better under a 250 watt or 400 watt metal halide . If you are using 250 watts and above, you can place the Crocea even deeper in your tank, or even use 20k bulbs. If using T-5s, make sure the bulbs have individual parabolic reflectors on them, and the clam is placed at the upper half of your tank to be ?safe?. Power Compacts will in a lot of cases NOT be adequate for Croceas.
In the case of T-5 lighting, remember only the 10k bulbs produce enough photo energy for PAR on clams, and the actinics produce little to no PAR depending on color temperature. It is also noted that the longer the bulbs are, the more penetrating power they carry. Thus a Crocea should be fine in a 20 inch depth under a 36 inch fixture. Having a Crocea in a 20 inch depth under a 24 inch fixture raises doubts, as most 24 inch fixtures only have 4 T-5 bulbs under them, with 2 of them usually carrying heavy actinics or some shade of high color temperature bulbs. In this case a retrofit is recommended to be able to cram in more lower temperature bulbs.
In short, you do not have enough light for this clam. I wouldn't keep a crocea under anything except mh (maybe t5 in a really shallow tank).
As for suggestions/advise...either upgrade the lighting quickly or get rid of the clam...and in the future might try reading what each animal requires before buying.
Here is some great info from reefcentral on clams
http://www.reefcentral.com/forums/showthread.php?s=&threadid=1072234
Lighting isnt the only requirement for them. You also need to monitor calcium, alk, salinity, mg, pH, etc...
More good reading for you...
http://www.reefcentral.com/forums/showthread.php?s=&threadid=1237978