"Mold is a sign of a moisture problem", for mold to survive it must have the right environment and a food source that is "Wet Enough Long Enough". To properly correct a mold problem you have to first fix the moisture problem, if you only treat the symptom (the mold) the problem will return.
The cabinet blocked air flow that would have helped remove dampness, this caused the drywall to be "Wet enough Long enough" but only in an isolated area behind the cabinet creating the right environment for mold growth.
Vacant house that did not have the heating or cooling system on for a long period of time. This caused excessive moisture in the air that saturated the paper on the drywall and caused mold growth.
Heated air from the ducts hitting the poorly insulated wall caused condensation behind the paneling and promoted mold grow. This was not found until remodeling was started, it was not visible during an inspection.
Hot moist air from a bathroom vent fan was discharged directly on the roof sheathing , this isolated the moisture and kept it "Wet Enough Long Enough" for mold to grow.
Hot humid outside air hits the cold duct work in the crawl space and causes condensation in the summer. This will also form on the wood members under a home year round. This can cause a musty smell in your home from the soil and wood being constantly wet.
This is a flat laid heavy plastic membrane, 3 mil or heavier, covering all open soil. This is the minimum advised for damp spaces and is not advised for spaces that get standing or flowing water at any time. If water works on top of the barrier it will act like a "swimming pool liner" and hold water under your home for extended periods.
This is where the entire space is covered with a membrane that is sealed to the foundation higher than the level of the soil around the home. Any surface water seepage through the foundation will go under the barrier and back to the earth. Old ground vapor barriers need removed prior to encapsulation.
All piers and seams in the membrane are sealed tight to create one large barrier so water cannot work its way through in any location. Any water that works up from the ground will be contained under the membrane and eventually be absorbed back into the soil.
All pipes and any other penetrations should have the membrane sealed around them to keep water from working by. Duct tape does not work in this environment and will not hold long term.
There may be the need for a sump pump to be installed. This would also require an actual sump pump pit to be installed, 5 gallon buckets are not large enough and can cause the pumps to wear out prematurely. These should be installed in the lowest area of the crawl space.
As noted above and below, condensation can form under a home due to temperature and humidity changes. Sealing vents helps to stabilize the crawl by keeping outside air from freely flowing under your home.
The pictures below shows damage and mold growth on the Band joist around the perimeter of a home, this is the most common location that moisture related issues are viewed in crawl spaces. This is a weak thermal point where condensation forms due to temperature and humidity differentials, over time the wood breaks down and mold forms. On this home the structural members were damaged which caused the floor to drop two inches. Repair for this could include the items listed above along with cleaning and treatment of the affected areas with proper chemicals, bleach should never be used. Also repair or replacement
of the damaged wood and installation of foam board insulation between the floor joists over the band joist, or expanding spray foam is ideal, this will prevent condensation from forming. Fiberglass insulation should never be used in this location because it will act as a sponge and hold moisture against the wood.