Doesn't look like root rot to me, looks fine and healthy.
Depending on the media you use, some light brown sediments may be seen. As long as it does not get DARK brown you should be fine. Dark brown or black is root rot.
Not sure what brown spot you are talking about, but the roots look great. If they do start to brown/blacken you can trim them off, or move the plant into dirt
Doesnt look like root rot, is there any slime? If so use some h202 at 2 to 3 tsp per gallon of water and try to keep your rez cool if there is.. Cheers