Just removed big bed from a spare bedroom and this is what I found (and I'm worried)
It may be just surface mold. Bleach & water it like joecaption said and see if that takes care of it. I did that at my mother's house and it was fine. She was running a humidifier 24/7 in the room where she had the problem and the surface mold had started in a walk-in closet where the clothes were packed in tight. We got rid of the old clothes, aired it out/cleaned it up, reduced the humidifier use, and it's been fine ever since.