Are you seeing mould growing on the bark of your trees? This is actually called lichen and it is made up of two organisms, fungus and algae that grow together as one organism on the surface of the bark of trees.
Tree lichen grows best in cool, partly sunny, moist locations.
Lichen on tree bark is completely harmless to the tree itself. The rhizines (similar to roots) allow them to attach to the bark but do not go deep enough to harm the tree in any way.
If you’re not sure what is on your tree, call us to take a look.