If a basement wall spot appears to be mold, there's probably a moisture problem, Marston says.
"Call a soil engineer or home inspector to help determine where the moisture's coming from," he says.
If water is coming from leaks in the interior water pipes, the repair could cost thousands.
However, if the problem is poor water flow around the house, the exterior of the house may just need simple regrading, Marston says.
"The owner could hire a landscaper for a couple hundred dollars to add more shrubs and to make sure the dirt that's beside the house is 6 inches higher than the level of dirt that's 10 feet away," Marston says. This creates a hill for water to drain away from the house.
If you live in an arid climate, a wall spot may not be moisture at all.
Bennett, who lives in Colorado, says: "Out west, in our part of country, it's very dry. We don't see a lot of water filtration, so we don't run into mold issues often. So a spot could just be dirt."
The solution? Soap and water.
COST: Anywhere from a few dollars to several thousand dollars.