woolly aphids are quite large whereas mites are tiny. They're also greenish coloured and have themselves wrapped in that tiny cocoon.
If that is what you have then i treated mine with the wife's eco washing up liquid (saponins) and some cooking oil mixed and diluted in water. One spray wasn't much help so i sprayed them daily a few days and it did shift. It was a 7foot high espaliered red devil apple tree (12 foot wide espalier). It only had them that one year. My only other trees at that time were a cox and a conference both were unaffected.