It's lichen - and it's harmless to your shrubs. I've heard that it shows that the air is clean!

I think it is a symbiotic relationship between a fungi and an algae. Those must be some really 'old' twigs.

The yellow-green stuff coating your plant will be algae, but the green-greyish growth on part of it is lichen which has developed out of the algae where a fungus has combined with it. 'Symbiosis' means mutually beneficial. Lichen grows on masonry, with time, and is also grazed on by reindeer in the far north.

they are both species of lichen.

Yup both lichen

Oh, I wasn't looking closely enough. I can see them now. I was looking at the blurry bit. Just looked like slime.

Jonathan the twigs/branches don't need to be 'old' to have lichen form on them. One of our azaleas has had for over 20 years...

That's very interesting to learn. I've never had it occur on my own plants. But now that I think of it, I have seen them on grave stones where several different species live alongside each other, then their patterns are all the prettier.

Added to which surprised as I am lichen can damage the parent plant, but, so far as I know, only if it totally takes over!

thanks very much for your answers - is it ok just to leave it then?

Yes...no problem. :-)) Just keep an eye on it.