High light. If it's small (less than 3 inches) spot feed with phyto as they don't get enough nutrient from light alone. They may move by using water as a sort of jet stream if they are unhappy. I've found clams larger than 3 inches are pretty hardy and easy to keep if you keep stable parameters. The smaller ones are more difficult because of the added nutrient needs.
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.