Starfish - Although most are omnivores, I have *never* seen or heard of, in 6 years, a starfish, of any species, eating a clam. Even serpent stars that are notorious for catching small reef fish and consuming them have never touched a clam, other than to move over it, IME. I know of several reefkeepers who keep Blue linckia with clams.
As far as the urchin is concerned, some more info or even a picture would help alot. "purple urchin" doesn't tell us much, although you may be referring to a tuxedo urchin. As with starfish, there are some urchins that prefer to munch on vegies but there are several that are omnivores. Most sea urchins eat plant and animal matter, including kelp, decaying matter, algae, dead fish, sponges, mussels, and barnacles. Will a tuxedo eat a clam? Probably not. However, most reefkeepers don't like urchins of any kind because they can be bulldozers when it comes to knocking stuff over as they move about the tank.