American Shingle is a great product - it looks good, it has fantastic waterproof qualities, it lends itself to complex roofs. But it has one property that you need to be very mindful of - susceptability to damage from chemicals.
There's an ongoing lawsuit regarding American Shingle roofs and while I won't touch too closely on the details, the crux of it is that some roofs have been damaged by overly caustic solutions being spread all across them. The bituminous laminate has separated and the grit has largely let go.
As an owner of an American Shingle roof, you need to get ahead of the moss because any treatments applied need to be very, very gentle in nature. The roof simply will not tolerate a shock treatment dose.
If you find your roof has gotten a little away on you, patience and an easy-does-it approach is the right way to retrieve the original finish. It may be that you have to treat it, then treat it again in six months but it will definitely clean up. After that, you can step the treatments out as once the vegetation has cleared, the solution will serve as a very effective preventative.
I've treated a great many American Shingle roofs. I know what to use on them and how to treat them with requisite respect.
If you have an American Shingle roof and you want to keep its characteristic look and you do NOT want it damaged, get in touch with Green Agenda for the best advice and products available.