Hi Lorelei! Wet & Forget is a gentle cleaner that works over time with Mother Nature to remove moss, mold, mildew and algae. If the stains on the surface are more green in color, you should see results a few weeks after applying, if the stains are black it can take a few months for you to start to see a change. We'd like to look into this further and learn a little more, if you could please give our office a call at 888-727-8524.