Thank you for your question, Brian.  Scale insects have not been identified as a common pest of hydrangeas in the Pacific Northwest, but have been seen in hydrangeas in Colorado.  Here is their Extension publication about them:  https://extension.colostate.edu/topic-areas/insects/oystershell-scale-5-513/

If this is not the insect you detect, please send clear, closeup photos so we can help with control recommendations for it.  Thanks!

Have sent pic.  Not Colorado Scale

Thank you.  Although they appear to be scales, I am afraid I cannot identify the type of scale through the photos, but this Extension article has photos of several varieties as well as control recommendations:  http://ipm.ucanr.edu/PMG/PESTNOTES/pn7408.html

I hope this helps.  Good luck!