This is cottony camellia scale. It has been a big year for it as we have had many similar questions. We recommend using horticulture oil.
Here is our page about cottony camellia scale and recommendations for management, https://extension.umd.edu/hgic/topics/cottony-camellia-scale-shrubs
You can apply a horticultural oil spray (at the summer rate, 2%) now. You can find horticultural oil in garden centers or the garden section of some hardware stores. Read and follow the product label instructions.
Scale insects are notoriously difficult to control. If the problem persists, you can treat again with horticultural oil in the dormant season (after the leaves have fallen from deciduous trees). Pruning out some of the heavily damaged portions of the shrub is also okay.