4.9 
Growth
There is no way to know for sure without proper testing to know if this is mold or not. But the only thing mold needs to grow is a little bit of moisture and warmth. A crawl space is the ideal environment for this.
The rot
As you can see the floors are being held up by wood, cinder blocks, and an old jack. The wood floor joist is actually split and you can see it seperating.
Foundation settlement
This crack is probably a good indication of foundation settlement. Foundation settlement happens for a variety of reasons. The soul around and beneath the home compacts over time and with water it washes away. This creates pockets under the home where the foundation "settles".
