First question, what does it being purple have to do with it? Secondly, what species of clam is it? Derasa, Crocea, Maxima, Gigas, Squamosa? They all have different needs so it helps to know what kind you have. For instance, Some are sand dwellers and some need to be on the rocks. Some are very high light demanding, others aren't. They all need steady and high levels of Calcium and Alk. Magnesium also needs to be kept steady. As for feeding....some say that depends on size...so how large is it? Others say Clams don't need to be fed. They are filter feeders and eat plant life. So, if you're going to feed them, you need to feed quality phytoplankton. Keep in mind that when feeding phyto, you take a large chance of feeding an algae outbreak.
A lot more information is needed to properly answer your question.