Is this mold or is it something else?
It looks like a fungus to me see the light fuzz. Here in the PNW it is fairly common in dark and damp areas. I have seen this on carpet and Sheetrock many times. Even though our humidity is not extreme like back east we do have many mold and fungus problems because of our moderate temperatures and year round moisture. I find the least offensive method to kill it is a 3% hydrogen peroxide and water solution will kill it (use caution as it can discolor some materials even at this dilution) but it doesn’t stink like bleach. Note this will also take care of mold &amp; mildew you may find in your window tracks&amp; sliding door tracks that very commonly grow “gunkies” in our area.