Scrape everything off and wash it with detergent and water solution, that will get rid of it... That said in any case you should have some sort of mechanical ventilation in that crawl space to move the air around.

Thanks for the feedback. It's going to be a lot of scraping. It's even growing a little on the floor. What type of detergent do you use? I've read people suggest white vinegar or bleach. Don't really want to be in an enclosed space with bleach...even with a respirator.

We still don't have utilites to the property yet, so while the generator is running during the day I have a fan in the access door. Originally I had a weak box fan, but today upgraded to a fan that claims to move just under 4600 CFM.

After I get the moisture barrier in I'll have a dehumidifier or conditioned air in there.

What Virgil said, your builder should clean all that up before they do the foam...Or call ServPro and they will clean all that in no time.

But If you plan to DIY you can use Mold & Mildew Foaming Action Stain Remover without bleach, or Kaboom Mold and Mildew Stain Remover (this-one got bleach in it) but it's a No-Drip-Foam so it goes on easy and it stays without dripping... That said cleaning that is a PITA especially in the crawl space.

Good luck

Thank you! I am the builder, but I've hired a GC to do my framing, siding and roof...basically get us all dried in and the exterior complete. So the crawl space work is on me. I don't plan to foam the joists, but to put rigid foam along the perimeter of the crawlspace wall (on the interior). Unfortunately it's supposed to rain all week so it's probably going to get a little worse.

Call a water restoration company and have them do a treatment. You will need fans and dehumidifier going forward.