Moss tends to be attracted to shady situations. In terms of roof construction this will be where the roof doesn’t shine…very often. Normally on a slope with a north or east aspect.
In itself moss is not a problem unless there are large amounts of it, in which case rainwater drainage is hampered and this can accelerate deterioration of the covering over the years.
Careful scraping away by a competent roofer is the answer, with any damaged slates or tiles revealed then being replaced.
Trade systems involving cleaning with a high pressure hose can do more harm than good so be careful with what a salesman tells you. Concrete tiles in particular can be damaged reducing their effectiveness, and water can be forced between them and into attic spaces causing further disruption.