I have a well established big leaved hydrangea in my yard (over 20 years old). This year there are small, white fuzzy spots on the lower stems and under the leaves. Not sure if they are larvae or fungus.
White Spots on Bottom Hydrangea Leaves
Certified GKH Gardening Expert
Your problem may be powdery mildew. Here is an article or two that you may find helpful: https://www.gardeningknowhow.com/problems/get-the-cure-for-powdery-mildew.htm