How to clean my gray walkway turning black?

Use a high pressure washer.

We power wash our sidewalk every year.

Power washing is the best way. You don't have to buy one if you only use it once a year.....you can rent them at many hardware/home improvement stores.

Hi ! I purchased a product called wet it & forget it . It gets rid of the black mold / mixes stains . It works on concrete , stone , vinyl and other surfaces . On concrete , it doesn't work immediately , but over time . I see a big difference , and will give the sidewalks a second dose .