A closer image would be helpful, but from what I can see by enlarging this pic, it looks like Lichen which is not harmful and shows that you live where there is clean air.

On the other hand, large quantities of lichen may be a symptom of other problems. Less lichen can get established on actively growing bark. It may be a good idea to do a review of soil conditions, light conditions, moisture levels, and nutrient levels, to see if vigor can be restored.

Thanks