Hello Red Reefet,
Until your clam is ~3" long, it can't get enough of its own food through photosynthesis - it's mantle is just too small. As others have said, you need to feed the little guy (every other day IMO). In a 90-gallon tank, you will not be able to put enough phytoplankton in to do the job w/o risking your water quality. Here are a couple of options:
1. make yourself a feeding hat (search here on RC for "feeding hat" and you'll get wonderful DIY instructions plus pictures"
2. put the clam in a small bowl of tank water, keep it warm and put the phytoplankton in the bowl (remember to only use enough to just tinge the water green - too much can choke the little guy). It should take about 20-30 min. and you should see the water get clear.
I've successfully used method #2.
Best of luck,
Cheri