Algae Problem in DWC?

I have a DWC setup under a 600 watt hps in a 68 liter rubbermaid. It isn't lightproof, but up until now my roots have been looking white and healthy for the past couple of weeks. A day or two ago I topped my res and upped the nutrient level to about 1000 ppm. Today I checked the roots and there is a dark brown sludge covering the roots that are submerged in the water (see pic). Is this algae? I added some H2O2 to the res just now. Will this fix the problem, or should I do a res change?

Thanks.

i would lean more towards root rot, make sure your res is light proof, and look into some h202