I visited the certanteed shingle manufacturing site in MA a few years ago.

They recognized the algae problem years ago and now include a small amount of copper granules in their mix to prevent it, so you won't see this in their newer shingles.

I think they have been doing this for 10 years. If anyone knows the
exact date of this process change, we could tell how old (or at least age)the roof is, by the algae on it?