Most candy's and brains have feeder tentacles that they'll extend when they sense food in the water. Some are more likely to feed than others. If you can squirt a little food in the water and coax the corals to send out their feeder tentacles, then just turkey baste some brine/mysis/krill/etc near them and they should grab the food and bring it to their mouths. I've found some LPS to be voracious eaters while others are more ho-hum about it.