What should I do about mold in a new build?
First of all, you should probably contact a mold abatement contractor.  Not only should they be able to remove the existing mold, they should be able to identify its cause as well.

Then you may need to consult an attorney since from what you've said, this doesn't seem to be a one-shot deal in your unit but a systemic problem likely due to defective materials and/or workmanship.  So the builder may be liable and the city, who presumably inspected this work, may be liable too.

But it certainly doesn't sound like a DIY project.  Note also that it may not be safe for you to continue to live there until it's resolved.  I'd take the advice of the mold contractor on that.