That is a scale insect, most likely cottony camellia scale.

We don't often see these on hydrangeas, but moreso on plants like hollies, yew and camellias.

Scale are sucking insects. The white fluffy elongated part are the eggs that the mother scale (the darker oval) has laid. She will darken and fall off this month and the immatures will hatch (tiny crawlers which you can watch for by putting out double sided tape if you like.)

Here is our page on them:

https://extension.umd.edu/resource/cottony-camellia-scale-shrubs

Dieback is not common but the sticky 'honeydew' substance they secrete can have a sooty mold grow on it which blackens the leaves- again, not killing the plant but unattractive in heavy cases.

You can put on gloves and squish/remove the eggs, or just keep an eye on the situation, especially if you see beneficial insects like ladybugs helping to control (by eating) them.

Other control options are covered in the link above.

Christine

You are welcome.