It looks like green hair algae (very common), and diatoms (also common). Your best best is several large water changes a few days apart. 50% or more. 50% will reduce nitrate and phosphate by half each time, so it take a few large ones to bring it right down. The problem with test results is it only shows what's in the water. The algae IS feeding on something, and it's common that is sucks it up faster than you can measure it (that's why people grow cheato in the sump, or use algae reactors, or algae scrubbers). Essentially, this is what's going on in your tank, but it's happening in the display, instead of in the sump. Reduce feeding, and lighting. Do big water changes to get rid of nutrients in the water column (from rotting food, fish poo, etc.), and you will get it under control.