Looks like a cottony scale.

You can use direct hits of insecticidal soap,diluted according to directions,the repeat as needed. If you don't want to use that, a harsh spray of water to the underside of the leves may work.

In the meantime, figure out what has gone wrong for the hydrangea, then fix it -- is it too much or too little sun? Too much or too little water? Or what?

Thanks, the only thing that has changed is that bug. These are old mod sized bushes that have done very well every year until now.