Most lawns will contain moss at some time or other, but you can reduce the spread by a few simple steps.
Here in the UK we love our close-cropped lawns, but they provide the ideal environment for moss. Simply raising the height of your lawnmower blades will work wonders. As will regular aeration, cutting back of overhanging plants/trees and improved drainage.
Moss can be killed and even where the problem is severe, an area can be reclaimed. I will be happy to advise you on a suitable programme for your lawn.