I'm seeing a similar thing in my DWC. It's a light beige color though rather than orange or green. It is on the walls, and on the roots of the system. I haven't changed the water in about 3 weeks. Will do that this weekend. it's not 100% void of light because of the room around the rock wool in the baskets.
I am using a flat tupperware tote that is about six inches deep and just under 4 feet long. I spray painted the sides black to black it out.
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