For the consumer, there is nothing worse you can do to a roof. For the record, do not ever let anyone on your roof with a powerwasher.The granular surface of the roof is designed to take the heat, UV rays, and rain, and it is designed to slowly wash away a little at a time over the years, but you do not accelerate this process by powerwashing.
To remove this fungus, it is recommended it be gently scraped off the roof with a putty knife, and then cleaned with a soft brush and a very diluted solution of bleach and water. Call a roofing contractor who will gently clean the roof, not blast the stuff to oblivion with a powerwasher.
If you have trees growing over the roof, trim them back. You don't want branches falling onto a roof as they can cause damage, and they also help cause the fungus issue. The roofing manufacturing companies have been working for years on a solution to the staining that you sometimes see on a roof, but they haven't figured out what will work in all situations.
These roofing contractors however have figured out how to get more jobs, how to decrease the lifespan of your roof, and how to assure themselves of continued work, so be careful who you hire.
Ray Wilson is a Home Inspector, covering all of Long Island (Nassau and Suffolk), the 5 boroughs of NYC and now upstate NY. For any home inspection related questions he can be reached at Meticulous Home Inspection Corporation. He can be reached at 631-902-6761.