Have you a picture of the Cotoneaster? It would help us help you.
Very often a greenish yellow coating is lichen which is harmless, but without seeing it ,we are a bit stuck.

the greeny yellow stuff does sound like lichen which as Owd says is not going to be a problem.

As for berries mine hasn't many this year but I am putting that down to the wet weather we had when it was in flower. The wet meant fewer bees/butterflies were in flight to pollinate the flowers and this means fewer berries now.

but a photo would help.

two photos submitted any help to identify problem please

Definitely lichen so not a disease or a fungus. It just shows you have reasonably clean air.
Lack of berries may well be down to lack of pollination in spring.
There is the possibility though that after 26 years the shrub has reached the end of its life span. It does happen.
You can check if the branches are still alive, by gently scraping a little of the bark away. Live branches have green under the outer lay, dead ones are just brown.

thanks ever one for information m