Never spot feed a clam - you will suffocate it. Put the phyto in the water column and it will filter it. Depending on the light it came from, move it up as it adjusts to your lighting. Especially if you have a halide.
If it's not used to intense light, you will bleach it and kill it. There are some great books out there regarding Tridacna care. James Fatherree and Daniel Knop provide excellent reading about these amazing creatures.