If you are talking about yellow sun cup corals, they do need to be fed--and each one individually, because they are actually separate little corals all bundled together, and they feed singly--not collectively. I can't discern if that is what you have, but eye-be-dam is right.
Start with swirling some cyclop-eez in the water and wait about 10-20 minutes. We use the frozen kind. This stimulates the sun cup corals to open up. Then, use a dropper or baster to feed each polyp as well as you can with the brine or mysis. They will only accept food if they are flowering.
Also, they don't like light--so try to keep them in a cave--and you will notice that they will open up when your lights are off. They are beautiful--and are worth the extra trouble. We only target feed them about once a week--they also like marine snow--and other micro liquid food like that--but you don't have to target feed that.