Hi Karen, 

This looks like Japanese Maple Scale (Lopholeucaspis japonica), a non-native type of armored scale insect that can be difficult to manage. Their waxy body covering shields them from predators and certain pesticides. Scales remove plant sap and high populations can cause premature leaf drop, branch dieback, defoliation, and ultimately tree death. 

Mechanical Control: Use a high pressure water spray or gentle scrub brush with water to remove scale covers from tree trunks to improve aesthetic appearance and/or reduce populations. Prune out dead branches with scales. 

We have a resource on Japanese Maple Scale where you will find additional control methods if the scrub brush or water spray does not control the population. https://extension.umd.edu/sites/extension.umd.edu/files/publications/FS-967%20Japanese%20Maple%20Scale.pdf

Jamie