What is this moldy looking stuff on our trees?

This is lichen. It grows on the bark, but it does not harm the tree or take any nutrients. UT Extension Horticulturist Celeste Scott talks about why it is growing on the trees and recommends not trying to remove it.

Related Videos:
Lichen on Trees
What can I do to keep lichen from growing on my azaleas?

Related Resources:
Lichens are harmless to trees
Do not bother removing lichens from landscape