Yes!  Cedar shake shingles can be safely cleaned!     Cedar shakes are a natural resource with an insulation R Value of 4.  And when properly maintained, this luxurious roofing material can last a lifetime.  Left to the elements however; and mother nature can and will reclaim them in short order.  We have personally observed a cedar shake roof in McLean that was only 15 years old but was in need of replacement.  This was largely due to a heavy tree canopy above it, and years of leaves and debris left to accumulate, hold moisture and accelerate decay.

Similarly, moss and lichen grow naturally of cedar shakes, especially on northern or shaded slopes.  Shakes have a natural wicking, or capillary action, and tend to draw moisture in to them.  Moss and lichen substantially exacerbate the problem by providing a near continuous source of moisture.  Think of it like a wet sponge on the butt end of the shake,  providing constant moisture until the cedar shakes ultimately fails from decay.

Keeping leaves and debris from the roof can be addressed during routine gutter cleanings.  Moss and lichen however;  present a delicate sitation best left to professionals.