I have a red Acer tree (about six years old)up the trunk there are signs of white power with brown spots. I have brushed them off and my tree appears to be healthy. What could this be?
Probably horse chestnut scale by the sound of it, which is common on Acer, particularly Acer palmatum varieties. If the tree is very large, its usually not possible to treat, but it seems not to kill the tree anyway. On smaller trees, its relatively easy to deal with because its on the woody parts - its not uncommon to find it also on the pot (if its potted) or on a wooden fence behind the tree. I just use disposable cloths lightly moistened in methylated spirits to rub them off, then wipe over the treated parts with a wet (with water) cloth afterwards. Info and image here https://www.rhs.org.uk/advice/profile?PID=650