Its is difficult to diagnose the problem with just a couple of photos. A deeper inspection should be performed to determine why there is mold.

Mold means that there is moisture where there shouldn't be. I would be very suspicious of a leak somewhere. I would bring this up to your builder if he is not already aware.

"A deeper inspection should be performed to determine why there is mold."

@Bryan The Builder USA Inc., 6 month old post and since new construction should all been resolved one or another by now.

It must be resolved because I have not seen a "Mold on the Drywall After Construction" discussion.

Holy Cr@P ! That must be the fastest mold to ever grow on a brand new home/construction, especially if it was painted too. Six months late but I would have definitely gone up in the attic to inspect for water damage via uninsulated AC vents or a drippy air handler up there., If there is a 2nd floor above then inspect any windows present over that area for poor installation or leaks. Would like to know the outcome, mold growing that fast is crazy.