What do you call 'stringy stuff" on trees?

There are many tree lichens out there but this book nails the exact one

Boy, do I love these books. The North Woods Naturalist Series books are a must for anyone interested in the outdoors of our region. Published by Kollath+Stensaas of Duluth, Minn., this series of field guides are strictly about Northwestern Ontario, Northern Minnesota, Northern Wisconsin and the north half of Upper P. Michigan.
The guide shown above is Lichens of the North Woods.
Other titles I have are: Amphibians and Reptiles, Orchids, Dragonflies, Spiders, Moths and Caterpillars, Insects, Butterflies, and finally, Ferns and Allies. The full titles are the names just listed followed by "of the North Woods."
Each is written by a regional expert. For instance, Lichens is authored by Joe Walewski, director of Wolf Ridge Environmental Learning Center in Finland, Minn.