Certified GKH Gardening Expert
The mold is likely lichen. It is not killing the tree, but rather a symptom of it dying. Lichen are especially attracted to trees that are sick and dying already.
What kind of tree is it and what sort of symptoms does it have? We may be able to diagnose it for you.
There are a few things it might be. The first is that it is dying of old age, especially if it's an ornamental. Ornamental trees have a rather short lifespan, 10-15 years normally. Another may be a fungus disease (it would not be visible like the lichen is). You can treat the tree with a fungicide, like neem oil, to help combat that. There may also be something attacking the roots, but on a large tree, it can be difficult to check the roots. Check around the trunk for any signs that there is damage there to indicate if there is a pest in the roots.