that's a myth about the smaller clams needing supplemental feeding. i know that from experience. i grew twelve 1" clams to about 2.5" with just proper parameters and lighting.
do not squirt anything in the clam. you could kill it very easily like that. if you feel the need to feed it, you could prepare a container of tank water and put the phyto in that and put the clam in that container for a little while, so it doesn't foul your water.
you should also move the clam to the bottom...and slowly work it's way upwards to higher light. like corals, you can shock them with too much too soon.