Not mold, it's salt! I've seen that commonly on coco and rapidrooters even- you actually get salt crystals building up as the nute solution evaporates off exposed bits!
Feel it... a bit hard? Crunchy, even?
Keep an eye on your plants for signs of stress, fert burn, lockouts... include a flush in your feeding schedule every so often... and beyond that, don't worry too much!
That shit creeped me out to the first time I saw it and then I was like, oh f'real, it must be time to flush!