This occurs on damp soil, usually in a shady area, if the soil has not been cultivated for some months or longer. Its needs digging over, which will improve drainage and get rid of the moss.

Just quit now 'mate' Nobody wants you. Sorry about 'mate', Paul Smith.

Sometimes you can make a virtue out of a problem, since moss can make a good ground cover in difficult areas. On the other hand, algae isn't so useful. The way to tell the difference: Moss is fuzzy, but algae is just a green stain or crust on the soil.

Is your soil very compacted? Moss grows very readily for all the above reasons as well as compacted soil which Bamboo has intimated. Now, if you desire that area to be green, leave the moss if grass will not grow there as has been mentioned too.