Need more info or a photograph - where is the white stuff, on the leaves or on the trunk/stems? Is it dangling and like fur? Or is it a light powdery covering? Is it like cotton, on the trunk and woody parts, with a bit in the middle that's not white?

We had `white stuff` on our apple tree last year,( only 2 years old)...turned out to be woolly aphid, by examing patches under a magnifying glass you could see the aphids wriggling about. Got rid of it by washing affected areas with soapy water, but sadly never got any apples!