The tiny world
With a little help from Tom Walker and the marvelous Brodo key I'm now tentatively satisfied that this little lichen is Lecanora strobilina, Mealy Rim-lichen.
The little yellow circles, rimmed in mealy green, called apothecia, the reproductive structures of the lichen. Many kinds of lichen have apothecia that more or less resemble these.
Down in the apothecia, spores are formed as a result of sexual reproduction. The spores can disperse and develop into new lichens.
But the apothecia are so tiny you would not see them just by walking up to a twig and looking at it with the naked eye. The largest apothecia in the photo are less than half a millimeter wide.
The whole lichen patch
In fact, if you did happen to notice the lichen patch on a twig as you walked by a tree, it would look like a bit of fairly smooth grayish green paint. In the photo, it's the round-bottomed patch in the middle. Keep in mind that the twig is smaller than it looks in this photo.
However, under the microscope that patch of Lecanora lichen is a whole world.
And it knocks my socks off that these tiny organisms are living lives so unseen.
If someone has differing opinions on this lichen's identity, I'd love to hear them. You can email me here.
— Diane Porter, Fairfield, Iowa, February 9, 2022