Our main bed is pushed against the corner in our room. There's a very small amount of space between the mattress and the two walls. We removed the frame and the headboard so that the box spring sits on the floor. We did this so that it is easy and safe for our kids...so they can climb in an out easily.
My wife and I both sleep so that our head is on the wall side. We've noticed that black mold would eventually appear on the wall on the side of the mattress where our head is. (I clean it up with bleach and alternate with vinegar)
However, the other side of the bed that is against the wall, does not have this mold problem.
(Note: both walls that our mattress is touching are exterior facing walls and they do not have insulation, we live in Southern California, we live a block from the beach)
What could be the cause? What is the best way to prevent this?