Ours is a 2 storey house which had one of those long television aerials attached by a bar to a chimney and no attachment to our television for many years. Every bird known to man perched there at some time or another and each left their calling card which helped our roof and the roof of our next door neighbour to become an elevated live moss garden.
I attached a paint scraper to the end of a brush handle and climbed to the apex of the roof and six inches at a time began to scrape the moss off both sides. Ensure you're not afraid of heights and observe safety regulations. There's no problem getting to the roof; the problem is getting from the roof back to the ladder. If you "freeze" you're in great danger of panicking and falling or at best have the fire-brigade service rescue you.
But, the scraping is easy. Just take your time and get all of the moss off. Don't leave residue and you won't have regrowth of moss. Incidentally, if you still have a disused television aerial attached to your chimney, bring a hacksaw with you too.