This Camellia appears to have a heavy population of scale, a sap-sucking insect. Large populations can be challenging to control, and will require repeated and vigilant treatments to be effective. We cannot clearly see enough detail to determine which scale this may be, so below are two pages of information on the two  main groups - soft scale and hard scale. Between them, the approach to treatment is essentially the same (as given at the bottom of each page), though knowing which species it is will more easily inform when to spray as their life cycles can differ. Otherwise, you will have to more closely monitor the stems for the emergence of the vulnerable juveniles ("crawlers"); you can wrap pieces of double-sided tape on several stems to trap them as they move about, making their detection a bit easier, though they're still quite tiny.

https://extension.umd.edu/hgic/topics/soft-scales-trees-and-shrubs

https://extension.umd.edu/hgic/topics/armored-hard-scale-trees-and-shrubs

Miri