The last thing anyone wants to see on their roof is algae discoloration.
This is a very common occurrence throughout the Southern and Eastern United States and is better treated with prevention.
The discoloration does not mean a defective, but it can be difficult to remove and is best left to a professional.
Pressure washing may be effective but it is often very powerful. Without experience in pressure washing you can unwittingly remove shingles in the process of the washing. Only a trained professional should use high pressure cleaning on your roof. In some cases, they might also use a bleach mixture to remove the stains.
The roofing contractor may decide that the best way to remove the algae is to install metal stripping on the roof. These metal strips oxidize when they come in contact with the air. When this process happens, they release metal ions. When it rains or if water is applied to the roof, the ions will travel and helps to break down the algae.