Sorry for being late on this post but I have to agree totally with Pat. I too have had this happen, especially in my terrarium. I find I need to repot spagnum quite often (twice a year) if it gets heavy and rotten. I use Thiomyl systemic fungicide as a deterent for these conditions. I see NO critters like mealies or fungus spores or anything other than signs of rotten spagnum. Sorry to disagree with those thinking otherwise.
Tin, if this were mine, I would not treat like Pat does (that will definitely cure the problem but seems like swatting a fly with a bazooka). I would cut off the rot (the roots DO NOT look healthy to me, sorry!) Treat with your favorite sterile agent - some like cinamon, others charcoal, I like Thiomyl. Then let dry a couple days (rather than water, why not try the spag n bag?) Then when the cuts look OK, I'd repot mine in fresh spagnum (loose) that has been soaked in KLN rooting compound. I would NOT put back into same environment until it proved to me it was worthy
Good luck, whatever avenue you choose.