This photo illustrates a good example of mold colonization on drywall.  Each circular shaped dark spot you see in the photo is actually a separate mold colony.   The spaces between each colony are due to the mycotoxins that are released by each colony as it grows to protect itself from encroachment by the other surrounding colonies.  It is these same mycotoxins that can be harmful to human health, and why it is important to remediate mold growth in Tracy as soon as possible.