Hi

This looks from your pictures like a blight (fungal disease) called volutella buxi but your mention of the white substance makes me wonder if it could have a pest, so I've included two articles that look helpful. 

Here are recommendations from the West Virginia extension for both management and a fungicidal control if it's the fungal blight:

Disease Management

Prune out infected branches and destroy them by burning or hauling away in a trash bag. Pruning dense shrubs also can help in air movement, sunlight penetration and keeping canopy humidity low. Sanitize pruning tools by dipping in 10% bleach or 70% ethyl alcohol between each cut. Do not prune when foliage is wet. If possible, remove infected fallen leaves and litters.

Alleviate plant stress by providing irrigation and fertilization as needed. However, it is better to irrigate through drip irrigation and avoid overhead sprinkler to keep foliage dry. Do not use horticultural fabric as a mulch to suppress weed growth. It is better to use bark or compost mulch that may help retain moisture and promote root growth.

Fungicidal Control

A combination of chlorothalonil (trade names include Bravo, Daconil or Fung-onil) and thiophanate methyl (Cleary 3336) can provide good control of Volutella blight if used preventatively or at the very early stage of symptom expression. Removal of highly infected stems by pruning before fungicide application can improve the efficacy of the products. Other products that are registered for the control of Volutella blight include propiconazole, copper and mancozeb.

You can find the complete article here: 
https://extension.wvu.edu/lawn-gardening-pests/plant-disease/landscape-tree-disease/boxwood-blight

And here is information for the pest: 

https://ask2.extension.org/kb/faq.php?id=789483

Thanks,

Catherine

Master Gardener Volunteer