Crawl spaces are the hardest place to keep mold gone because they are naturally damp, dark, and poorly ventilated. Removing the visible growth is straightforward; the lasting fix is a sealed vapor barrier and humidity control so the wood stays below the moisture level mold needs.
It is also a health issue for the rooms above. Air moves upward from a crawl space into the living space (the "stack effect"), so crawl space mold and its spores end up in the air your family breathes - which is why containment during removal matters.