Fruit Tree Problem (Help Please)

We have a silvery fluffy looking stuff on the bark of some of our fruit trees, it can be rubbed off but it keeps spreading.

It is mostly on the plum trees, and a bit on the apples.

Can anyone advise please.

Thanks for the answer, done a Google search and it's not wooly aphid.

After searching more i found out it's LICHEN, so i don't think i need to worry about it.
Unless anyone knows different ?

Thanks again.

Jilly.

Thanks JayG, yes the air here is very good. we live in the country and have no polution.

We purchased a piece of land near our house and after clearing away the brambles we found an old orchard.

The trees have lots of blosom and the leaves are coming out, so hopefuly now they can breath again we should get some fruit 

Re good for fungus, maybe we should try mushrooms!