Mold can grow inside of humidifers that are not properly cleaned. However, this may or may not be the cuase of the mold you are seeing in your attic area. Mold on the underside of attic sheathing is more likely being cause by a lack of proper attic ventitaltion.
If there are any supply ducts in the attic, check to see if they're leaking, which can lead to condensation and mold growth. According to The Money Pit's indoor air quality expert, Jeff May, you also should be sure that there is adequate ventilation in the attic space, and that there are no openings through which house air can leak into the attic. However, the mold may have already been growing there, due to excess moisture from other sources: a bathroom venting up into the attic, or moist airflows entering the attic around recessed light fixtures in the ceilings below.
If you already have ridge and soffit vents, do all you can to reduce or eliminate the sources of house airflows into the attic. As for the mold growth itself, small areas can be HEPA vacuumed (a HEPA vacuum cleaner has filtration that prevents the release of particulates in the exhaust and should, in my opinion, always be used for even everyday cleaning) and spray-painted to contain allergenic and irritating dust. Always wear at least a NIOSH-rated N95 mask or respirator when cleaning mold. For more information, see www.health.state.ny.us/environmental.