Although clams CAN feed, none of the things listed on this thread are likely going to do any good. Freshly grown Nannochloropsis or live yeast might be the only thing that they can both effectively catch, digest and get nutrition from. Scraping the film off of your front glass with a magnet scraper will probably do more than adding any commercial food. It costs a clam energy to filter out stuff that is the wrong size - net negative for the clam. If they are properly feeding from particles in the water, they will have stringy poop coming out of them several times.
I cannot get a read on this clam... sorry. What does it look like when it fully extends?