azalea affected by a grey mould
Azaleas in question are more than 20 years old and have lichen-like grey mould on small branches. There are no leaves on affected areas, no sign of insects. Mature azalea has been taken over by disease and now no flowers. Cause please.
A photo would aid in my identification but I will give this my best shot. If it was in fact lichen, the lichen would not be the cause of your azalea's problems - lichen are harmless. For more information on lichen, please visit this link: https://www.gardeningknowhow.com/ornamental/fungus-lichen/treatment-tree-lichen.htm
Scale pests, which would stress the azalea and affect leaves/blooms, are sometimes mistaken for lichen. To give you an idea of what I mean, check out the following photo: http://nwdistrict.ifas.ufl.edu/hort/files/2014/03/crapemyrtle-scale.jpg
The grey mold could also be black sooty mold. Black sooty mold is mold that grows on the honeydew secreted by scale and other numerous insects, such as thrips, which are hard to see with the naked eye. However, given the lichen crusty appearance, I am leaning towards scale.
Here is more information on scale and black sooty mold to assist you in zeroing in on a diagnosis: