Your corals, all of them "soft corals" as they don't have calcified skeletons, are photosynthetic. The algaes, the zooxanthellae that live inside them (also the stuff the white witch in the lion, the witch, and wardrobe uses to create turkish delight and hot cocoa), thrive off light, like plants. Some people feed their zoos mysis and such things, but not everyone does. I feed mysis as well as cyclopeeze and phytoplankton to my tanks and the soft corals catch a little bit, but it's probably not necessary.