I am suspicious that this is not a fungus but an insect - either mealybugs or a scale insect.
Both of these insects may congregate on the stems of boxwood and give the appearance of a gray mold or fungus - especially the mealybugs.
Look closely at the stems and see if you notice little legs or any movement. If it is scale, you will see little bumps on the stems.
Spray your boxwoods with Bonide All Seasons Oil according to the label directions. This is a horticultural oil that should kill these insects by smothering them. If it is scale, their bodies will remain on the stems even after they are dead so don't be alarmed if you still see them after spraying.
In the spring, you can apply Bayer Advanced 12-Month Tree & Shrub Protect & Feed according to the label directions. This is a systemic insecticide that controls many insect pests including scale and mealybugs.