It is mold. You will get this forming in your plastic containers more often than your wooden humi, cuz the plastic seals much tighter. Also, aim for a lower rh%, more like 63 than 70.
Just clean out your containers, refill with new kitty litter......and........BE SURE TO OPEN your plastic every 3 days to breathe going forward. Otherwise, the mold will return. It is a high maintenance hobby.