This is either mold mildew or algae, this grows on most homes and should be treated with soaps and detergents. Do not hire a pressure washer to blast it with pressure. Applying high pressure to your siding can cause permanent damage. These are organic growths and must be treated the correct way. Most times handyman, contractors or landscapers are not equipped to get this off properly. Hire a professional company that specializes in this type of cleaning like ourselves. Give us a call today and we can provide you with a free estimate over the phone.