Mould grows in the right conditions, mostly damp and with a bit of a food source, but that maybe nothing more than air contamination oh and did I mention damp?
It's been a pretty damp warm autumn, ideal conditions for mould growth, so you perhaps need to look first to see if any moisture can be getting onto your cover board. The warmth from the bees cluster will keep the inside of the hive dry, your mould spot may be too far away from the cluster for that heat to have any effect, so your mould spot could just be from excess moisture in the hive or too far away to dry, rather than anything getting in.
Check to make sure there are no visible leaks.
I would question why you are looking anyway, as I'm kind of of the opinion that the bees have been put to bed for the winter, we've dealt with the nasties (for now) we should have fed enough to last through the winter, no good can come of sneaking a peek every now and then, all you are doing is worrying yourself and allowing a blast of cold damp air into the hive.
Frisbee