Does the mold seem more attracted to dark-colored manuals?
In my old house (which was very close to a body of water), we had severe problems with some kind of light-colored mold/mildew that was preferentially attracted to dark colors. Fortunately it only seemed to grow on the surface of objects, and didn't seem to take root or bloom/flower, so it was easy to wipe it off with a paper towel that'd been dampened with a mild cleaning solution.