Hi - This looks like a type of soft scale, likely cottony maple scale. These are insects that feed on the plant sap, which can result in yellowing and dieback symptoms if there is a large population left unmanaged. However, if you are only seeing a few of these, the best thing to do would be to pick them off by hand to remove them from the tree (if they are reachable). There are beneficial insects in the environment that may show up as the season goes on to help control these pests naturally. You can apply a dormant rate of horticultural oil during the dormant season (when the leaves fall off the tree in the fall) to control overwintering scales. Here is our web page with information about soft scales,
https://extension.umd.edu/hgic/topics/soft-scales-trees-and-shrubs#insects

Christa