For landlords, understanding how to respond to suspected mold and how to educate tenants about prevention is essential to protecting both your investment and your reputation.Avoid Using the Word “Mold” PrematurelyWhen a tenant reports dark spots or moisture issues, it’s important not to refer to the substance as “mold” unless it has been professionally tested and confirmed by a certified mold inspector or remediator.Until testing is complete, landlords should refer to the issue as “black stains,” “discoloration,” or “moisture-related staining.”

As to microbacteria, are you in a high moisture area?

I look for moisture around radiators/baseboards, bathroom fans, GFCI outlets, window guards, fridge door seals not cracked (tenants will never report this, your electric bill will), under-sink P-traps for slow drips.

If they don't dischage at LEAST a few feet away from the foundation they could be putting more moisture down there even if it doesn't show up in liquid form.     

A recent case showed 78% indoor humidity and wall studs at 25% moisture content.

Also, check to see if they installed crushed stone and a moisture barrier below the slab.Lastly, it would be good to know if a licensed plumber did the underground.If you actually have a true 7.5' ceiling that would be great for a Chicago 2 unit, but before you do anything consider pulling a zoning certificate and a foia request...either you or your friend could do that.Happy to hop on a call and talk more about the deal.

Basements around here can have radon/moisture/egress challenges, so I’d test and mitigate as needed before ever housing a tenant - now or after I move out. 

Mildew near AC vents can result from excess humidity, so your advice to reduce moisture and increase airflow was spot on.

Once the grass sprouts the grass mat will also help keep moisture in that area.Who says you can't get high school kids to work. 

.– Foundation/moisture: Older homes sometimes don’t have proper drainage.