High pressure strips the granules off an asphalt shingle. Those granules are the shingle’s UV protection and wear layer, and once they are gone the roof ages years in an afternoon - for a problem that was only ever growing on the surface. Anyone who offers to pressure wash your roof should be sent away.
What we do instead is gentle manual removal by hand, followed by a low-pressure or no-pressure soft wash carrying a treatment that kills the moss and the black algae streaking at the root, so it does not simply grow back from what is left in the seams. Asphalt shingle, cedar shake, tile and metal each get their own method and their own handling, because what is safe on metal can crack a dry cedar shake and what is right for tile is too aggressive for shingle.
Moss is a roofing problem, not a cosmetic one. It holds water against the shingle, works under the edges, and lifts them - and when that trapped water freezes and expands, it breaks the shingle outright. Treating moss early is cheaper than the roof replacement it eventually causes, and the black streaking is algae, not dirt: it will not wash off with plain water and it comes back without a treatment.
