It's just lichen, won't cause any harm, but if you find it unsightly, you can wash it off gently with a solution of horticultural soap and water, or eco washing up liquid diluted in a lot of water.

Yes I agree its lichen. I think it gives the tree or shrub character. Harmless though :)

Thank you Bamboo and Pam 24 for your help as you say it is not unsightly so I will leave it Many thanks.

Supposed to mean the air's clean - otherwise it wouldn't grow, lol.

Thanks Bamboo that's good to know I will enjoy the fresh air out there now.