The most common type of stucco stain comes from something clear that's also used to treat stains: water. In Florida 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.
One of the most effective ways to clean hard water (yellow) 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.
Other sources of stucco stains, ranging from mold, soot and paint to vining plants, grasses and shrubs, can be removed with one or a combination of several methods:
chemically -- with specialized cleaning agents for stone, lime, cement and stucco or with diluted bleach (if tolerated in a sample test of the area to be cleaned)
with water pressure -- using even spraying with a home garden hose and attachment or with a higher-pressure spray system
mechanically -- with brushes and sandpaper or other abrasives and muscle power, detergents and sponges
Whether using a wet or dry method, stucco cleaning should most often be a gentle process. Saturating or pre-wetting stained areas with water will help to draw stains to the surface, and once the pores are full and discoloration is closer to the top, removing problem areas usually won't involve getting rough with the stucco and damaging the surface.