this is one of the scale insects. If the infestation isnt too bad scrape them off and squish them. Spray the plant with a systemic insecticide will also work. if you want to be environmentally friendly then sparya with soft soap.
they are sap suckers and can allow other infections in to the plant.

looks like scale insect to me, i scrape mine with a stick and spray a bug killer on them. I had it on my acers this year, although i;ve never had it before.

Scale insects. Scrape them off or spray early to mid summer with malathion.

Thanks for your very quick responses. Everybody is in agreement as the problem and the solution. Great work.

Yes, Cushion Scale....as above.