Lichens attach really well to shingles and do harm to the grandules. Find a roof cleaning company and have them apply a solution to the area where the lichens are to kill then lichens. Let Mother Nature take the lichens off. You do not want to put a lot of pressure on the area where the lichens are or else you will damage your shingles. Good luck

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.

Lichen on a roof will soon be followed by moss. Using a product such as Bio-Shield or another organic moss killer in a hose end sprayer will work if you're persistent. Avoid pressure washing - shingles will be badly damaged even with a lot of care.

Once the roof is clean, a strip of copper or zinc (there are some precut versions available) installed along the roof peak will prevent re-growth.