If you have good lighting, they really don't need to be fed. But if you really want to, you can thaw out any type of small, frozen food such as plankton, mysis, brine, whatever and feed them that. You'll have to wait until after your lights have been out a while or do it early in the morning. They mainly extend their sweepers at night. This is your best chance to give each head a piece of food if you're going to do it. When the tentacles are out the furthest. Keep the food small though, and keep in mind crabs, shrimp, etc.. will quickly rip the food out of them before the coral can ingest it.
Once it's used to getting food, if you can hold food near it during the day, so they sense it, without something taking it, you can sometimes coax the tentacles out. Keep in mind if you happen to touch any of it with the feeding tool or your fingers, the tentacles will retract and it's not going to eat. You can only touch the food to them. I've found it can be sort of tricky.
I like feeding mine. They're one of the neatest things to watch eat. They swell up quite a bit in order to ingest the food.