Q: I did a spring trim of my Scott Wallace yews and found a surprise under the needles. What is it and how do I deal with it?
A: This is cottony taxus scale, aka cottony camellia scale. The insects attach to the needles and suck sap from the yew.
If it is a minor infestation, as it seems you have, you can control it by clipping out infested branches. You could also consider spraying thoroughly with a horticultural insecticidal oil.
If the infestation is more widespread, it can be controlled with a systemic insecticide. Imidacloprid (click for sources) is the most common active ingredient.