How big is your clam? Per the literature, clams over approximately 2" rely mostly on photosynthesis to create their own food. At that size they get very little value out of phytoplankton and other micro-organisms. Yes, phyto etc. can offer your clam a boost, but its not doubtfully eating as much as you think.
Larger clams, again over ~2", do fine with a little bit of nitrates, or none at all. You are more likely to see issues with SPS when you have nitrates before you see it in your clams.
Croceas are just as hardy if not more so then any other clam in my experience. They adapt better, grow faster, and acclimate better then any other clam I've had, and I have had a lot of clams.
Your Tek fixture is just find for a crocea!
Do you have any additional photos of your clam. It honestly looks to me like it is is just further developing the colours in its mantle from the picture above.
However, given your description is sounds like part of the mantle is becoming spotty & transparent. Does this sound familiar? This is the result of zooanthelae either dying off or being expelled from the clam. You don't have to do anything in this case, its pretty normal and does occasionally occur from changes, like a move or a new system. The zooathelae will recolony and repopulate the tissue returning the colour to your clam.
One last thought; you and your LFS might have different lighting systems, thus making the clam appear differently in your tank then theirs. Same goes for depth and placement, etc.
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