Hi,
That looks like scale inects. They are softbodied sucking insects covered by a hard shell. If you crush one dark or red colored liqued comes out. Pretty yuccky. If it is scale, you need to get rid of it, it will weaken and ruin the tree eventually. There are different sprays including dormant oil and insecticidal soap that will help. Do some research or consult a local nursery to see what is recommended in your area. Japanese maples are sensetive to certain kinds of sprays, so make sure whatever you chose is ok for them specifically. Hope that helps
Steve