first, a power washer will make the problem worse. With improper pressure and upward motions will cause one, leaks, two, tear up the shingles.
I would scrape the shinges with a scraper or wire brush. Any too bad just replace the shingle.
When I do this for customers, I always use a product from sherwin william's called "woodscapes". its made for shingles, cedar and is a stain/paint. It comes in many colors and can be customed mixed also. With the nature of cedar shingles, it has to be sprayed on. doing this by hand is futile to say the least.
This stuff is very good, and it does soak up alot on the first application.