No NPK+micros and the tank has quite a lot of light and CO2? My very first GW siege a decade ago was worse than yours and after blackouts and water changes it turned out the tank was running out of nitrate. I dosed 1/2 tsp potassium nitrate daily for a few days and the GW vanished. Too bad that one hasn't worked since!
Usually it is triggered by an ammonia event though. Until I got my marvelous Poret sponges in the sump I was having regular issues with green water. Check your filter to be sure it isn't clogged? Modify your intake so you can put on a huge prefilter sponge for more bio?
Besides the UV you could go out and spend money on a diatomaceous earth filter. Lots of gizmos out there to help us out!