This looks like cottony camellia scale on the holly foliage.  This insect has one generation a year.  This is a sucking insect which exudes a honeydew as they feed and fosters the growth of a black sooty mold.  
Most likely the crawlers have settled on the foliage now. 

You can spray with a summer rate of horticultural oil.  Do not spray in high temperatures and follow label directions.  Coverage will be difficult as you will  have to target the upper and undersides of the foliage and bark.  This can knock the insects back but will not control them. Dieback is not common.
In the late fall (October when trees have lost their foliage) you can spray with a dormant rate of horticultural oil.  

You also have the option of contacting a commercial landscaper as they have insecticides that cannot be applied by a homeowner. 
Here is our website https://extension.umd.edu/hgic/topics/cottony-camellia-scale-shrubs

Marian