Congrats on the maxima! Can you tell me what's the wattage/length/bulb configuration of your t5's? It looks good and healthy.
I haven't heard of cowries eating clams before. My guess is that the clam was on it's way out and the cowrie finished it off. I often hear that shrimp eat clams - not true either, unless the clam is dying. I have skunks, coral banded's and peppermint shrimp in my tank and never have (or have had) issues with shrimp.
I often hear this, "clams need to be fed phyto plankton" - not true. Once tridacnid clams reach about 1" in size, they no longer require phyto feedings. Clams do require minor trace nutrients to grow and thrive. Aside from the regular water chemistry parameters necessary for keeping sps corals, your tank should have traces of NO3 and PO4. This can be achieved by feeding your fish regularly, and your fish pooping - yes fish poo is the perfect food source for your clam(s).
Make sure you keep a higher SG. I prefer 1.0255-1.026 (refractometer), although clams can/will tolerate lower SG's and can even be freshwater dipped to remove parasites such as PM.
Here's some good reading on tridacnid clams.
Hope this helps-