The reason your water is coming off white is white paint. As this happens with many fiberglass tops. You need to repaint the top and seal it from the weather. Your top is mildewing because of moisture buildup due to the porus open paint/fiberglass. After a few years fiberglass tops colors and paints and sealers deteriorate due to ultraviolet rays, fiberglass, its coatings do not hold up as well as the rest of the vehicle. Just use a brush to clean it, no need for solvents exspecially ones containing bleach.