I see topics on a regular basis about mildew growing on composite decking materials of assorted brands. The first thing to remember is that nothing is maitenance free and for sure if it is outside. So if you have to wash your house and deck to remove mildew once a year that is not unusual and in my opinion a very good idea. Also mildew will grow on your car if you do not wash it and I have seen it on many outside units on central air/heat pumps. I have gathered this information over my 30 years in the paint business.