I suspect that this may be an “infestation” of barklice, a harmless insect; psocids. These insects eat lichens and spin a webbing over them as a means of protection. They can be “blown” off the bark with a strong hose blast of water if necessary. Here’s a link to a column on these critters: Barklice
If you want a definitive identification, you will need to contact your local Extension Office and inquire about their plant diagnostic services. Cooperative Extension System Offices
