Scale insects feed on plant sap (more or less...they are a sucking type of pest, not chewing), and they are sorted into subgroups like armored scale and soft scale. The latter can be abundant and a nuisance on certain trees, but generally don't cause serious damage. Armored scale, however, can cause premature leaf shed or even branch dieback if numerous. Gloomy Scale is a kind of armored scale, and you can learn about them and what to look for on the bark in the linked page.

Management of scale (refer to the bottom of the first linked page) often involves the use of pesticides, since their waxy turtle-shell-like body covers shield them from lots of spray options. A mature tree will need treatment by a certified arborist (or anyone who holes a pesticide applicator's certification) who can apply insecticides in a more thorough manner than most home spray equipment can manage. There are different insecticide options you could consider, but successful suppression of a high scale population might take well over a year to achieve.

Scale insects tend to preferentially infest trees under stress, even if symptoms of that stress isn't yet obvious. Street trees, for example, are common victims of scale, having to deal with the limited root space, drought stress, reflected heat from the pavement, road salt or other pollutant accumulation, and so forth. Trees not growing near a road could still be stressed by conditions like being over-mulched or planted too deeply, and the girdling roots that could result from this (maple being a species prone to developing them) could slowly "strangle" part of the canopy, making it easier for scale to overwhelm a tree's tolerances. Therefore, prevention of scale outbreaks tend to just revolve around the usual tree care and prevention of stress -- watering when needed, not over-mulching, avoiding soil compaction in the root zone, avoiding root zone flooding from sources like a nearby roof downspout outlet, and so forth.

Miri