Lighting can be a fuel for algae. You are not seeing as much nitrate because the algae is consuming it pretty quick. If it was me I would first try to pull out as much by hand and then rubber band a toothbrush to a siphon hose and scrub your rock and siphon it out while doing your pwc. Do two 33% pwc's a week and run your lights less than you are now. IME
Ps remember it did not happen overnight and it wont go away overnight. Patience is a virtue.