This sounds like colonies of mildew. These microscopic organisms are part of the natural life cycle of organic matter, including decay and composting of landscaping plants and mulches. Excessive moisture in flowerbeds, shrubs, mulch, and other organic materials creates ideal growing conditions for these organisms. Prevent, or minimize it by eliminating the excess moisture: be sure irrigation systems are not directed against walls and keep plants trimmed back to allow ample light and air circulation. Remove these discolorations from stucco surfaces with household bleach, followed by a thorough flush with clean water.
