I'm not sure anything is wrong with your roots. Can't really tell from the picture. Are your nutes organic? If so that is probably your harmless brown slime.

Like the last guy said about the buckets. But just buy black buckets at your local hydro store. Like 5 bucks a piece.

And if you choose to cover the top of your net pots (and their are different schools of thought on that), then dont use garbage bags, go buy a couple of feet of panda film and cut them into squares with a hole in the middle. The white side will reflect light back upward.