Answer: Moss and algae usually grow where it is damp and shady. If you can open up the area to more sunlight and provide a quick way for water to runoff the walk so it dries off quickly, and also keep it swept clean, that should help prevent or slow the growth. Improved air circulation can also help. An annual or twice yearly power washing should also help dislodge the growth plus any spores along with any organic matter helping them thrive there. The products that you might use to clean it off of say a roof (such as bleach or zinc chloride) would be likely to run off the walk and damage surrounding desirable plants, so it is difficult to recommend one.
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