Question
My flowering crab tree has almost always had been 1/2 covered with this green scale growth on the bark of the trunk and on the branches.  Doesn't seen to hurt the tree but I don't care for the appearance.  Can I get rid of it or leave it alone.  Thanks for your time.

Answer
These are lichens. A lichen is an unusual organism composed of a fungus and an alga living together symbiotically. The alga converts sunlight and carbon dioxide in air to food. The fungus surrounds the alga, protecting it from drying, and lives off the food it provides. Lichens appear as green to gray-green leafy or crusty growths on the trunk and branches of trees in poor health. They are totally harmless and are in no way responsible for the poor health of the tree. The reason they are associated with declining plants is that as woody plants loose vigor and decline, the number and size of leaves decreases. This allows more sunlight, which lichens need to grow, to reach the trunk and branches.

Prevention and Treatment: If plant health is restored by correcting the real cause of decline, leaves will increase in size and number; less sunlight will get to the trunk and limbs; and lichens will gradually disappear.

I would fertilize the tree and keep the tree in good health and not worry about the lichens.