Sounds like horse chestnut scale, which is often an issue on acers. It's usually only on the woody parts, not on the leaves - if your tree isn't huge, its possible to remove them by hand - use a jeycloth or similar, dampened, and moistened slightly with methylated spirits, but don't get it on the leaves. You may need more than one disposable cloth if there are lots. Wipe over with a clean, wet cloth afterwards.