No, Diatoms are actually small microscopic silicate organisms. They are quite awesome to look at under a microscope due to their symmetrical crystalline structures. But definately not phytoplankton. There are creatures that do eat diatoms though, such as gobies, blennies, crabs, shrimp, CUC in general. But for coral that need some sort of direct feeding you should get phyto or zoo plankton. Or if they'll take it like some will, food pellets.