What they are
The dark streaks on shingle and tile roofs are mostly algae that feeds and spreads in heat and humidity. Lichen and moss can follow on shaded slopes.
Why not pressure wash it
Pressure strips shingle granules and can crack or loosen tile. It can also force water under the roof covering. A roof should be soft washed: solution applied without pressure, given time to work, and rinsed.
What to expect after
The streaks usually fade the same day. Heavy lichen can keep fading over the next rains as the dead growth washes away. Shade and trees decide how soon the growth comes back.