Moss is a small plant that prefers shady, moist areas. Moss is not generally harmful, but on cement and other hard materials, it creates a slippery surface. Moss also holds onto dirt and debris, which can make cement surfaces look dirty. As moss is basically just a very small plant, you can kill it by removing and discarding it. After that, simply use a common product to kill any remaining living pieces of moss and prevent its return.
Pour biodegradable soap or detergent in a clean 5-gallon bucket. Use the detergent or soap amount (it depends on how concentrated the product is) per gallon as indicated on the product label. It's not a problem if you add extra soap or detergent.
Fill the bucket with water from a garden hose or an outdoor spigot. Dump this soapy mixture directly over the mossy cement. Use as much water and soap as you want--you really can't use too much.
Scrub the cement with a stiff-bristled broom. The moss should start coming right up--if it doesn't, add more soapy water and keep scrubbing. If you own a power washer, you can use that instead of the broom, but make sure you don't use too high of pressure and that you hold the nozzle several feet above the cement surface or you may remove some cement along with the moss.
Rinse the cement using your garden hose or the pressure washer. Put on rubber gloves and mix bleach with water in either a spray bottle (for small areas) or a garden sprayer. You can buy these at garden centers. According to experts at Oregon State University, use 1 part bleach to 1 part water, spray it on the surface and leave it for at least 30 seconds. Rinse it off with your garden hose or pressure washer.