The vast majority of corals don't consume phytoplankton because they're carnivorous. However that doesn't mean there aren't zooplankton, pods, and isopods that will consume it and, in turn, be consumed by the corals.
It's definitely possible to over-feed. It depends on your source. If it's Kent Phytoplex, absolutely possible just like any other "dead" food. If it's DT's, still possible, though they are more or less alive so they will persist in the water column longer. If you're growing your own, it's real fresh and will last a long time, but you can still overfeed.
