I do agreed plankton is good food for corals (based on my experiences), but some long time Reefer disagree. I use it and it doesn’t increase PO4 in my tank. Quite the opposite the PO4 has decreased.
I suspect you are referring to RedSea Reef Energy AB+ and that is amino acids, good source of N for corals. Amino is good to use if you don’t have enough N due to lack of fish or snails and it is important nutrient / building block for the corals. From my research the Red Sea formulation is quite diluted so there are alternative products on the shelves that are economical long term. I prefer AMIN from Fauna Marin but be warned I been accused of marketing on R2R so do your research. I use AMIN and little goes long way in my system and I only comment on things I have used vs. things I have not used.
Happy reefing,