The white on the undersides of your holly leaves are the ovisacs of cottony camellia/taxus scale.  Each spring this insect completes development into an adult, mates and then produces hundreds of eggs in these ovisacs.  Lady beetles, lacewings, and other predatory insects then eat the majority of the eggs or early instar crawlers of this scale species.  The remainder survive until the next year when the cycle repeats itself.  Crawlers have already settle for the year and have started producing a waxy covering that protects them from insecticide applications.  A systemic insecticide could be used to possibly provide some control, but the active ingredient would need to be a neonicotinoid such as imidacloprid or clothianidin.  Both of these products are suspected as having impacts on pollinator health.  The scale insect rarely does long term injury to the health of the plant but can produce sufficient amounts of honeydew for sooty mold to grow on which will make the plant look unthrifty.  A 3-4% horticultural oil application can be made during the fall by spraying the underside of the leaves and the branches of the holly and this could provide some control.  It is too late in the year for contact insecticides to provide substantial control of the pest.  The oviscacs typically take a year or more to wear off the underside of the plant's leaves.