I've seen this a few times and never knew what it is. This particular specimen is on one of my maples (Burgundy Belle, to be specific). What is it?

Some kind of scale, maybe?

We get it on our redbuds.

That is woolly aphids as we know them, common on Acers (Maples). Use some white spirit and a paintbrush and that gets rid of them!
Simply dab the white spirit on, and it dissolves the waxy coat of them. Let the birds and the sun do the rest! Do not get it on the leaves!
A maple for you!
Regards.
Neil.