deck is painted
A bleach solution of bleach & water
water with soap & bleach.
Power wash or mold mildew remover spray
I would be very careful using a power washer on a painted deck. More than likely you are going to remove some paint and that is not going to make you happy! I would use a bucket of warm sudsy water and a garage broom/brush to scrub and then rinse. Do this on a hot sunny day so that things dry quickly.
We powerwash with commercial cleaners and that works with our decking and vinyl rails. Having a waterfront exposure seems to hold moisture on our deck and need frequent -sadly yearly maintenance due to moisture and mildew. The deck and rails can have moisture droplets each morning.
There are multiple effective choices at the hardware store. They all work well. Btw If you don’t have a pressure washer it can be done with a push type brush and a bucket of prepared solution.
I think Ive used them all without a particulair preference.
Sometimes scanning the label can influence my choice.
I use the Behr on the Behr stain sealed decks to maintain existing stain product integrity and warranty. We have a soild stain applied-thats similar to a painted finish.
Krud Kutter —Behr cleaner—and Ive also used a Scotts outdoor cleaner plus Oxi Clean product. Our deck has changed with complete replacement I am seeing more of a mildew issue.
A bucket full of warm water with some bleach mixed in should cut through the mildew. Scrub it with a stiff plastic or straw bristled broom and rinse it thoroughly with clear water. Good luck :)