Certified GKH Gardening Expert
that lichen isn't harming anything. It can signal that there is something else that is harming your tree, though.
Depending on if the tree behind it died of old age (Those don't really live all that long) or of disease it could be something that you may want to take a look at.
I would treat with a fungicide to be safe. This article will help:
This article will give you more information about the lichens that you are finding on your shrub: https://www.gardeningknowhow.com/ornamental/fungus-lichen/treatment-tree-lichen.htm
This article will help you with hydrangea care: https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/growing-hydrangeas-hydrangea-care-guide.htm