If possible, a clearer photo would be helpful, but based on the host plant, we suspect this is a Cottony Camellia Scale (an insect with a wider host range than just camellia plants).

The simplest solution to avoid pesticide use would be to trim out most or all of the infested leaves. You could also prune out affected stems, but this may affect future flowering, depending on what variety of hydrangea you have. Otherwise, a low-impact pesticide, horticultural oil, is recommended. Two windows of opportunity present themselves with oil sprays to suppress Cottony Camellia Scale. In June, the crawlers (juveniles) hatch and are quite vulnerable to horticultural oil treatments, though more than one spray will probably be needed to get any eggs that hatch late. In winter, dormant stems that may harbor overwintering older juveniles can be treated again, and the oil concentration can be slightly higher (called a "dormant rate" or "dormant oil") since there is no foliage to damage and no active pollinators to avoid. Any time oil is used, though, it must coat the plant surfaces as thoroughly as possible - especially leaf undersides - since it cannot work on insects it does not contact. This is why cutting back growth may be useful as an initial step since it will greatly reduce the number of surfaces left to treat for the summer-timed sprays. Do not make applications of horticultural oil during temperatures above 85 degrees, and follow label directions about how to dilute (if needed) and when to re-apply.

Cottony Camellia Scale

Christa

Thank you for the additional photo and good observation! What you have there is not Cottony Camellia Scale but looks instead like a type of planthopper. There are different species that produce white waxy "fluff" to prevent desiccation. They can be found on a variety of plants and are not considered a major pest of Hydrangeas. It's good that you are now seeing fewer of them; you could simply wash off the rest of them with a strong spray of water from a garden hose. There is no need to use horticultural oil on these.

These pages provide additional information.
Planthoppers | UMD

Flocculent Planthoppers Arise | Ohio State

Christa

You're welcome.