This looks like sooty mold residue atop of honeydew, a common occurrence in areas with Spotted Lanternfly populations. (They produce more honeydew when they are adults, which is the life stage they are in from about midsummer until frost. You can learn more on the linked pages.) There are at least a couple lanternflies visible on the trunk in the photo, and there could be more up in the canopy.

Although it can be an aesthetic nuisance, honeydew isn't harmful, and the sooty mold that grows on top of it isn't a threat to plant health because the fungus does not infect plants or cause disease. The lanternflies do not need to be treated or eradicated (it wouldn't really be possible to get rid of them anyway), and they will die out once we start to freeze overnight in autumn. Any honeydew and/or sooty mold residue left by then will weather away on its own.

To protect tree health overall (unrelated to any lanternflies), we suggest circling the trunk base with mulch so the lawn/weeds don't grow too close to the bark, which would be damaged if hit by a mower or string trimmer (and such wounds are sometimes serious or fatal for a tree). A ring of mulch only about 2-3 inches deep, extended out as far as you'd like (but at least a foot or so) and not piled against the trunk itself, would be helpful.

Miri