If you use a magnet scraper to clean your glass that should suffice in feeding your clam, although a weekly addition of a few caps of phyto to your tank wouldn't hurt either. If you over feed a clam you can clog its gills? that it uses to siphon particles out of the water.
You want the colored side facing up towards the light where the zooxanthellae living in the tissue of the clam can use photosynthesis to help feed it. Lighting is the key issue with clams, I have about a 2 inch maxima I've had for over a year with NO feedings and is doing great --- under 250watt metal halides. I also have a nice crocea and about a 5 inch deresa. Also be sure your calcium levels are up 400 - 450 and an alk of 7 - 12.
I know this is after the fact but a little research goes a long ways before making a purchse