A few years ago I noticed the white scale patches on the reverse side of my hydrangea, 'Harlequin'. When I called the local Cooperative Extension Agent he told me it was cottony maple scale and the plant didn't need intervention - it could handle the attack by its own defenses.
From the front no one would ever have noticed it had the problem:
I did cut it back anyhow and haven't had the problem return. Since we have over 100 hydrangeas I was fearful it might come back to haunt me but my fears were unfounded.