First lets talk about why it is there. Moss grows naturally in a shaded humid area which gets some indirect sunlight with a sprinkle of foreign vegetation. Such as leaves and branches from the tree that overhangs the house. Another reason might be is lack of insulation in your attic is causing some extra humidity to your roofing members.
In most cases i tell my clients to simply have it cleaned up. You can do it in few different ways, the old fashion way is bleach and water. The other is buying a specific chemical which you can find online for about $20 to $40 and it will cover up to 1000 sq ft. There are some chemicals out there that are so easy to use all you have to do is just spray it and leave . And don't forget to remove all the leaves and vegetation off your roof as well as cutting back those overhanging branches. Your roof will thank you for it.