I have a lot of gray to black foliage on my Taxus some of which is dying or...
Your yew has a scale infestation. Here is information on what scale insects are, what they do, and how to control them.http://extension.umd.edu/learn/armored-and-soft-scales-trees-and-shrubs
The scale on your yew is cottony camellia scale (which aren't visible this time of year but will return next spring if untreated). Click on that photo for specific advice on that scale.