I'll add a few more things, if the sun ever comes out conditions will be less favorable for it and the plant will most likely outgrow it.
By the sounds of it, it's just a generic surface mold hanging on by threads and most likely can't spread significantly enough to do any major damage, possibly even a soot coming down with the rains, in any event I'm skeptical that it can spread faster then the plant can grow, seeing that you can wipe it off.
Assuming it's a fungus and If things get worse, there is anti fungal treatments you could use, but I'd be hesitant and probably wait it out as they can cause more harm then good and that plant will fair better from that sorta treatment once a little bigger.
How many plants are being effected by it and is that phenomenon appearing on the growth tips?