How big is the clam? Less than about 3 inches and they do require some supplemental feeding. Phytoplankton is good in that regard, but it's best to do it in a bowl outside of the tank.
Opening wide like that is called "gaping" and is a sign of stress. " gaping clam will appear as follows: shell fully open, mantle poorly extended, and inhalant siphon widely stretched."
I found this article that seems pretty good -- Caring For Tridacnid Clams
I've had mine for 3+ years now. It has grown from under 2" to 5"+ sitting on the sand under 150w MH
lighting. I did feed it phytoplankton once or twice a week untill it was close to 3".
You need good water conditions too, so please post more information about your tank.
? how many pounds
Sand bed? how deep
lighting? how old are the bulbs?
placement of the clam (bottom, top, on sand or on a rock)
all water parameters you can test for
age of your current setup (how long since it was cycled)?
what else is in your tank?