Clams over 3" do not need direct feeding when placed under strong lighting. I would caution you with the use of phyto as it can pollute your tank fast if over used.
Smaller clams do require feeding. However, spraying phyto at them is not going to result in any good. It's best to just add the phyto to the tank and allow the clam to perform it's natural uptake via filtering. A more labor intensive method is to remove the clam to a smaller, feeding vessel, and allowing it to remain their with the phyto in the water for a period of time. The latter method is more of a nurturing method and will require a lot more effort and may be stressful on the clam.