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You can also use a pressure washer to clean moss, lichens and algae from shingle roofs Sure you can...if you want to watch five years of the roof's useful life wash away. 1) as above, find a roof cleaning specialist (one that doesn't use a pressure washer) and let them have at it. 2) climb up, use an old fashioned corn broom, do the best you can and then liberally sprinkle the roof with detergent granules. Dish washing detergent is best but most folks use powdered clothes detergent due to cost. Make it look like the roof has been snowed on. But don't pile it up in drifts. Wait. Probably repeat a few times. 3) tell the insurance company to get a life. Its probably not the company itself but a keener, OCD agent. 4) open up the sun to the roof, there is most likely a tree or something that is both shading it and blocking airflow? You don't have to take the tree down or top it, just a spiral prune to allow more light and air movement. 5) If there is no tree...I don't know what to say. Birds must have "dropped" lichen spore laden droppings or something.

look like an old black knot .... what fraz says ... is endemic to prunus .. and there is nothing you can do about it .. that.. imo .. is cost efficient ... i dont tend to spend good money chasing a cure for a tree that is prone to one or both of these problems ... in other words.. im not going to spend 50 bucks.. including my time .. to save a 25 dollar tree ... in tree time.. it could kill the tree .. sometime in the next few decades .. and certainly.. if it starts driving you crazy.. and you just decide to get rid to it ... IDing the tree would be a great start ... ken http://www.extension.umn.edu/garden/yard-garden/trees-shrubs/black-knot/

The differing opinions above prompted me to conduct my own research. A fungus, an alga, and sometimes cyanobacteria living symbiotically are referred to as lichen. Whether their symbiosis is mutualism or parasitism in its nature is a matter of debate, since the relationship is fairly complex and not yet fully known. As I understand, the relationship is the following (in a very simplified way): the fungus, through its thallus structure and through release of certain beneficial fungi chemicals, provides its photobionts (alga and cyanobacteria) with optimal living conditions. Additionally, fungus absorbs minerals (mainly carbon and nitrogen) from the air and water, and makes them available to its photobionts. In exchange, alga converts carbon to simple sugars which the fungus feeds on, while cyanobacteria provides the fungus with nitrogen, converted to a form the fungus can actually use. Many view the above relationship as mutualism; others argue it’s parasitism, because the fungus consumes more than half the sugars produced by the alga, sugars which the alga also needs for itself. Whatever the case may be regarding the nature of the symbiotic relationship between the lichen-forming organisms, I think it’s generally agreed upon the fact that the lichen does not parasitize the substrate it lives on; so it won’t harm a tree or other thing it attaches itself to, because it doesn’t feed on that substrate. My question was whether or not anyone could tell me if it’s a moss or a lichen, and perhaps even identify it by its name. However, I’m glad the discussion took this turn, because I’ve now gained a great deal of appreciation for lichens. Still, if anyone happens to know the name of this particular lichen, please let me know. Thanks everyone.