Many of these white, splotchy, bark dwelling lichens sport various black spots or squiggles (apothecia in various shapes). These features can aid in identification, but ultimately it seems that a microscope is needed to make any kind of exact identification.
To make identification more difficult, there are also unlichenized fungi that look very similar growing on the bark of the trees
I suppose they aren't much to look at, but they do add a bit of contrast to the woods, especially this time of year in Minnesota. But what I think is interesting is that they are fairly common and easily seen, but largely unknown to most of us.