You might be surprised to learn that seaweed is a form of algae. Algae is a special kind of plant that, unlike most plants, have cells that are not highly differentiated. In other words, they do not have leaves, roots and branches. They can be a single cell or a multi-cell structure. They blow in the breeze and migrate everywhere. They are always around, it is just a matter of whether they are multiplying heavily or not. To grow to the size where they form a layer of green or black on your home, driveway or roof conditions must be right. Overall, algae produce about half of the oxygen found in the atmosphere.
Algae grows where it is humid or persistently wet. That is why you will see algae growing on the north side of houses and roofs or shaded areas of walkways and driveways. That is why you see algae growth across the shaded roofs of Marysville, Lake Stevens and Snohomish. Roof cleaning done with low pressure water removes moss and debris but is ineffective at removing algae. This is because algae is very good at affixing itself to the roof surface. To remove algae, the roof needs to be treated with a high pH caustic chemical. A diluted bleach or oxygenated bleach raise the pH of the algae's environment and cause it do die.
Zinc is another method of killing algae. Sometimes you will see an absense of algae streaks on a roof directly below a stack pipe or a chimney. Zinc coatings on flashing dissolve and run down the roof killing the algae. Some people put zinc strips on the roof in order to keep the roof algae free. While this method can work it does put toxic zinc into the ecosystem, especially streams.