01Look up
The roofline, and everything the gutter is holding.
NowAfter
By hand, not flushed from the ground.
What you are looking at
Moss holds water against the shingle and lifts its edges. Needle pack in the valleys stops the roof draining, so water goes over the front of the gutter instead of down it.
What Mark does
Moss lifted at low pressure, valleys cleared, gutters emptied, then every downspout run until it comes out clean at the bottom.
Leave it and the water finds the fascia, then the soffit, then the wall behind it.