Twice per day sounds like way too much and may drive algae blooms at some point. A light dusting of zooplankton with a turkey baster once a week or so is fine when you see the tentacles out on the brain. It can be beneficial & promotes growth. Depending on species & polyp size, meaty foods like mysis are ideal.
Most stony corals don't utilize fee floating phytoplankton, although it may be a food source for feather dusters, some pods, etc.
If you choose to feed lots of planktonic sized foods regularly, be sure you vacuum detritus and consider a filtration method that can remove the old uneaten portions that find their way to the live rock and sand.