Its a lichen. I don't know where you are, but in most of the eastern states I would expect it to be Cladonia (or Cladina in some books) subtenuis. One of the "reindeer lichens". One problem with growing it is that you can't really tell if it is alive. It grows very slowly. When it gets dry it gets very brittle and gray, but it softens up when it gets wet. Grows on soil usually, I'm surprised to hear you say it grows on trees.

It is indeed a lichen, but identifying them to the species level more often than not takes an expert, a chemistry kit and a microscope... There are probably many hundreds of species, if not thousands that appear to be quite similar. The genus Usnea, is often found on trees and tree bark.

Not all that bad. Not as many as vascular plants in most places. 800 spp. in Great Smoky Mts. National Park as opposed to over 3,000 vascular spp., for example. The great majority are crustose spp. - those things that look like paint spots on rocks and bark. Those can be tough to i.d. But the "macrolichens" in any given area - like Steve's Cladonia - are no harder than grasses and sedges. Sure, you probably need a lens. And you need a good specimen - not some sterile scrap. I'm pretty confident of my i.d. on this one. Has closed axils, and dichotomous branching (note the little "forks" on the ends of each branch). But Steve might understandingly be confusing it with other lichen spp. in genera like Usnea and Alectoria that do make little "tufts" on tree branches. They are an interesting group to learn and they are around at all seasons.