One sneaky but common type of stucco stain comes from something clear that's also used to treat stains: water. Depending on geography, many homes have hard, mineral-rich water running through their pipes that stains their porcelain tubs, toilets and sinks. This same water can stain stucco, which has a porous texture similar to hard surfaces used for kitchen and bathroom fixtures.
But how does the water from the plumbing effect the outside of a house? It comes from the water hose and from drain pipes used for overflow because the rain water mixes with the ground water of sprinkler systems and garden hoses. Stains come up through the soil and penetrate the lower stucco walls, and sections of walls hosed off with piped water can develop staining -- ironically, from repeated cleanings.
One of the most effective ways to clean hard water stains is also ironic: gentle water pressure can remove hard water stains. Pressure-washing with too much pressure, on the other hand, can damage stucco surfaces by removing too much of the outer layer and creating more rough and porous areas where stains can set in and build up.
Not all methods of stucco stain removal are all wet, though.