as far as i'm aware you don't need to feed it, they'll get what they need from the light you provide, however some people do feed theirs mysis every now and again. Mine won't accept it as food!!
You can try using a turkey baster or some kind of syringe to put a little bit of mysis and/or brine right in the mouth. Typically the branching type is more aggressive and eats better than the "wall" type