Scale is a sap sucking insect that resembles an oyster shell that either cover branches like the tree pictured above or on the foliage of arborvitae. They can be challenging to control for when their shells are soft, insecticides work; when they are hard they do not. For deciduous trees you can use a systemic insecticide like the Bonide Annual Tree and Shrub insect control. For evergreens it would be better to use a horticultural oil like Bonide All Seasons Oil or Malathion to kill the scales.
An few signs you have scale is if you see shells on branches (not normal), the foliage is sticky or there are a lot of flies hanging around you plants (this also is a sign of aphids too).