Hair algae outbreak.

[Greenmaster]

Hair algae is a symptom not the problem... you have high nutrient production (your tests will read low because the algae is eating it all.) You should look at other ways of reducing your nitrates (Macro algae in the sump with a light, cutting down the amount of extra food that is being fed, do more water changes, and/or use a reactor method like de-nitrate) After you have reduced the amount of nitrates available to the hair algae your CUC should be able to take care of the rest.