Sorry for the slow response, we are still short staffed due to office shut downs with COVID. . Looking at your pictures, we believe this is most likely Cottony Camellia Scale. We've been seeing the scale on quite a few Hydrangeas in our area.  (So you aren't alone) It's possible that your hydrangeas have been stressed by the drought conditions we have been experiencing and have become susceptible to attack.  

For management-cultural control:Scrape scale insects off plants by hand with fingernail or toothbrush.  Water landscape plants properly, as drought-stressed plants are more susceptible to scale insects. Avoid excessive nitrogen applications as this encourages growth in scale insect populations. Generally, apply pesticide products to target the vulnerable crawler stage before the scale covering is formed.

I've attached a link to a one page document that discusses how to manage the scale in more detail, including chemical solutions.. https://pnwhandbooks.org/insect/hort/landscape/common/landscape-scale-insect 

Hope this helps!