Just taken a look at your roots and they are "hella" white 😉
Question? Do you flush your pebbles before use?
I have the same discoloration on a few of my girls when I either forget to flush or do not flush the pebbles adequately.
I would not worry too much at this point. If it spreads and has a bad odour, I'd worry.
Adding some bennies will definately help paranoia.