Your nitrate & phosphate readings are only an instantaneous indicator. It's likely that you have a PO4
source that is being taken up by the algae as soon as it enters the water column.
Think of it as a trickle of organics. Your pellets work the same way; fueling bacteria that uptake organics which are then exported by your skimmer.
You need to prune the GHA in order to:
-remove stored organics (not showing in readings)
-remove the ability of GHA to reproduce & consume organics before it can be taken up by the bacteria generated by the pellets & exported by your skimmer.
Algae & Bacteria are both sponges in competition limited by the organics in your water. Right now algae is winning, probably because of sheer quantity.