I was thinking the same thing phospates/nitrates are probably sky high. Also might depend on if the paper covering the back is translucent or not.
So here are a couple of ideas. Modify the amount or type of food that is being used.
Next time you clean the tank, try this - Empty out all of the water from the green tank, and fill half the tank with water from the tank that isn't green and replace the rest with freshly dechlorinated water.
If that doesn't work, try buying a phosphate remover.