In the tradition of Western Washington, a larger number of the trees growing in the Bellevue Botanical Garden were covered with moss and lichens. The mild climate and relatively heavy rainfall makes for healthy moss growing conditions. As thick as the moss was in Bellevue, it is even thicker in the temperate rain forest on the Olympic Peninsula. In some places it is so thick, it is difficult to find bark on the trees.
As I have not studied mosses and lichens, I will not hazard a guess on what species we are seeing here. If you happen to know, let me know and I will post the names.
I just thought it was cool-looking.