You've nailed it: the growth is a type of lichen. And it's showing a fascinating growth pattern.

As you may already know, lichens are composite organisms that involve a symbiotic relationship between multiple organisms. Fungi provide the structure. They cannot produce their own food, but they act like scaffolding. Algae and/or cyanobacteria provide food through photosynthesis; they act like the kitchen.

The fungi involved with lichens in North America aren't plant pathogens; they don't penetrate plants to infect plant tissue. So, lichens cause no harm. Plants like the American beech in your picture only serve as a substrate, although research has shown that certain lichens tend to be found on certain trees owing to several factors, including compounds dissolved in the rainwater that runs down the trunk.

The ring-like growth pattern in your picture may indicate that the lichens lower on the stem are struggling a bit. The algae and cyanobacteria need sunlight to fulfill their side of the bargain. I'm speculating that the lichens lower on the stem may not be receiving sufficient sunlight to support continuous, mat-like growth throughout the summer when the tree has a full, shading canopy. The rings are produced by new lichen growth that manages to become established before the colony totally collapses.