How can I fix this moisture problem to stop mold growing?
First, I'd redirect the bathroom vent out of the home. Best place is on the gable side of the house, but straight out the roof would also work. Depending on how much condensation you're seeing, and where it travels after condensing, this could be the cause.

Then I'd do some detective work. Start removing the insulation on the other side of the mold. Look for signs of water damage and follow them back to their source. There could be lots of causes, including a leak in the roof, plumbing problem, a clogged hvac drain line, or condensation on uninsulated hvac ducts. Water can travel a long distance before it finally settles on a low spot, so search far and wide.