My 9 year old pot grown Maple has developed a grey coloring on its trunk and some stems. About a third of the tree hasn't produced leaves this spring and a lot of the branches have only a few leaves on them. Usually it's a mass of new growth. I repot every couple of years. It's in a big wooden half barrel.
Grey Mold on Trunk and Stems
Certified GKH Gardening Expert
The grey growth is likely lichen. It is not the cause of the trees problems, but rather a symptom. They tend to attach to trees that are sickly and unhealthy. This article will give you more information: https://www.gardeningknowhow.com/ornamental/fungus-lichen/treatment-tree-lichen.htm In regards to what may be wrong with the tree, I would take a look at the roots. You may have some root rot or the tree may need to be repotted. It may also be a fungus attacking the tree, so you should go ahead and treat the tree with a fungicide, which will actually also get rid of the lichen.