Hi Mary - I use bleach and water with some “Simple Green” which you can purchase at any Dollar Store. The bleach will help kill the mold spores but you’ll need to spray it occasionally afterwards from time to time. Be sure and mix it with water. You may want to check a small area as it may affect the color.
Mary...A cup of bleach and a gallon of water
Hi Mary. I'm Cindy. Make a bucket of warm soapy water. Add a quarter cup of either bleach or vinegar. Scrub the porch with this solution using a hard-bristle scrub brush. Then rinse with clean water.
For my brick patio or my TREX deck, I just use vinegar on a thin paint roller. If the moss is really clumpy I scrape it off first with a long hoe and then roll the household vinegar--works for a whole year & cheap too. For Algae, I used 'wet & forget' and it killed it completely-but will reapply this spring to make sure. I will probably spray wet & forget on some shaded plastic siding to brighten it also.
Using a power washer with the appropriate cleaning solution would clean it right up quickly. Lowes or Home Depot employees can help with selecting the correct cleaning solution if needed (or any hardware store).