Mold has to be one of the most fear inducing things in home ownership.

So don't panic. During construction, humidity is uncontrolled. Keep running your dehumidifiers and have the mold cleaned.

I am sure the builder is going to say that it has been pretty wet - depending on where you live and it isn't a big deal. This is likely correct.

Mold needs humidity to thrive. Control it and you will be fine.

Basements generate the most humidity in the first year as the concrete is drying. Typical modern basement does not need dehumidifier after 1st year.

You've done the correct thing. Running dehumidifiers in new construction is absolutely necessary. The wood used in construction is wet. It needs to dry.

In your case, the pipes are supplying cold water. The temperature differential results in condensation. You need to have these pipes insulated to prevent the condensation. Keep running those dehumidifiers.

You probably want to have a dehumidifier added to your HVAC system, too.

The mold on the PVC pipe looks pretty bad to me. I recommend asking a mold removal contractor look at it regardless of what the GC says. Framing lumber shouldn't have generated that much moisture unless it was green or improperly stored. I suspect there might have been mold on some construction materials before they were installed.

Mold needs darkness, still air, moisture and a source of food like wood. I'm not sure why it was able to grow so well on PVC but it shouldn't come back if the space is kept dry and ventilated. The dehumidifiers should drain directly into sinks. The pros will tell you all you need to know.

Those pipes are drainage and should not be all that cold. Insulating drainage pipes would be very weird. I would imagine $5 a foot would be enough to insulate it. You can buy delivery pipe insulation for less than $1 a foot and it takes seconds to install on exposed pipe - so I am figuring you could find larger sizes for 5x more.

Drainage pipes should carry water close to room temperature - certainly toilet will almost always drain room temp water, showers - mostly higher, dishwasher - higher. Just the short hand washing might be colder than room - but remember a decent amount is tempered to room temp in a trap.

You are right to be concerned. My health was destroyed by living in a house with mold. For some of the population, it is a real and dangerous health concern (check out Surviving Mold). The first thing to do is find the source. Then removal of the mold is necessary. Do not handle this yourself and get another remediation company's recommendation (other than the one your GC called) before deciding how to move forward.

Mold spores can't grow in a dry environment. Once it's dry enough, the mold will become inert.

The good news is your GC called the right people. The mold remediation people will know how to deal with this. They will have guidelines for your to follow to ensure there is no repetition.

My guess is your unit was not heated or air conditioned while waiting to complete the sale. Nor was there a dehumidifier operating. Moisture is the enemy.

I've been through this a couple of times. It's a question of dealing with the moisture and there is always some kind of expense.