white pests on my hackberry tree

There is a big hackberry tree in the yard, and the lower branches' leaves are geting a blackish- mottled appearance. It is infested with some tiny pest. They appear as a tiny white flyspeck, like a tiny cotton ball, all white, covered with white hairs. Not long ago it had bark lice, which turned out to be a good "pest", is this one good or bad?
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