Mold needs 3 things to grow; 1. High Relative Humidity, 2. Warm Temperatures, and 3. Organic Material.
In a crawl space, the wood substructure of your home and the paper backing of fiberglass insulation are the food source. The insulation and wood absorb the moisture in the air and allow microbial growth to occur. As the moisture in wood increases wood rot can occur beginning at 19% wood moisture content. In addition, this results in poor indoor air quality as approximately 50% of the air in your home comes from the crawl space.