It appears that your Mugo pines are infested with Pine needle scale, Chionaspis pinifolia (Hemiptera: Diaspididae). This is a common occurrence on pines. 

Suggested treatment - per a fact sheet written by Howard Russell and Jill O Donnell of Michigan State University Diagnostics Services is as follows:

"For homeowners and landscape management professionals, the least disruptive method is to apply summer horticultural oil spray when the crawlers are active. Summer oils should be applied at a concentration of 1.0 to 1.5 percent. A more aggressive strategy is to apply a spray containing one of the following insecticides: Sevin, cyfluthrin (sold as Bayer Advanced Garden Insect Control for homeowners or Tempo for commercial applicators), bifenthrin (Talstar for commercial applicators) and Orthene. Two applications may be necessary to manage heavy populations as female scales have a long egg-laying period."

Here is the link to the fact sheet that also includes information on the life cycle of the scale : Time to check for possible pine needle scale problems - MSU Extension

And a link to Missouri Botanic Garden : Pine needle scale (missouribotanicalgarden.org)

You may want to contact a certified arborist who will evaluate the health and vigor of your trees and suggest  treatment plan. Be sure to ask if there is a complimentary initial consultation and what all costs are upfront. Here is a link to a website that will help locate a certified arborist in your area: Find an Arborist (treesaregood.org)

I hope you find this information helpful. Thank you for using Ask Extension.