Meaty foods. Krill is good. It can be at any time of day.
However, I wanted you to know that they are mean little buggers and they wander around. (They inflate their bodies around the entire skeleton). If you have any other aggressive corals with sweeper tentacles near the floor of your aquarium, it might find it and if that happens, they will duke it out.
I had green star polyps, leathers, anthelia, xenia, mushrooms, zoanthids near the bottom of my tank. My plate never bothered them one iota. However, one day a clumsy Mexican turbo knocked my galaxea off of the rock work. I saw it before work and just leaned it against the base of my rock work. When I got home that night, I decided that I liked the way it looked there and left it. Within 3 or 4 days, the plate wandered over there and killed about 1/2 of it.
The plate had always wandered but never more than a few inches. I had it for ages. It actually walked across the entire tank for the first time after the galaxea fell. With my luck, the first time it wandered over to the other side of the tank was the time the galaxea was near the bottom.
It's slowly starting to recover though.