To clean stucco, the main thing is to use a softwash method that will kill the organisms beneath the surface, which causes the stains. Stucco or EIFS is easily damaged with a pressure washer, especially on the trim pieces. Sometimes the black streaks under the windows are an inorganic stain that will not clean. Similar to getting a stain on your shirt, it may lighten but may not completely clean like a vinyl siding. Regular rinsing of screens and cleaning window sills is essential.