Ask Mr. Smarty Plants

From our Native Plant Database, Quercus stellata (post oak):  "Its roots are extremely sensitive to disturbance. Susceptible to oak wilt. Not often used in landscape situations. Slow-growing and long-lived." While it is the most common oak in Texas, it almost has to be already growing on your property in order to have it. It is rarely available in nurseries because of the difficulty in transplanting it. All oaks have a long taproot which, if broken, usually results in the death of the tree within a few months, but the Post Oak is the poster child for that difficulty. More information and pictures of leaves, bark and acorns from Virginia Tech.

There are a number of the genus Quercus referred to as "live oaks," but Quercus fusiformis (plateau oak), native to Hays County, is the common live oak used in landscaping and found in the wild in central Texas. It is susceptible to live oak wilt and live oak decline when stressed by drought, so care must be taken to protect it from injury both above ground and below ground to prevent infection. Pictures from the Image Archive of Central Texas Plants.

In terms of susceptibility to Oak Wilt, this website from The Texas Oak Wilt Information Partnership How to Identify and Manage Oak Wilt in Texas classifies red oaks as most at risk and white oaks (which includes Post Oaks) as less risk. From this same website, Oak Wilt Spread, explains why the way live oaks grow, often forming "mottes" of clusters with intertwined roots, causes the live oak to be at so much risk. 

Probably the most readily noticeable difference between the two trees is that the Post Oak is deciduous, dropping its leaves in the Fall, and spending the rest of the winter with bare limbs. The Live Oak, however, is tardily deciduous, which means that it holds on to the bulk of its green leaves until mid-Spring, usually the middle of March, dumps the leaves all at once and within a week or so has fresh green leaflets appearing. 

Tillandsia recurvata (small ballmoss) is an epiphyte, which means it is an airplant, receiving nothing from the tree but living space. It is not parasitic; the fact that clumps of it are often seen on interior limbs of trees is due to those interior limbs usually being the oldest, and declining from lack of sun caused by the other branches in the same tree; ball moss is fine with shade. Pictures and more information from the Image Archive of Central Texas Plants. From Floridata, ways to spread it but not to stop it. We have known of it being used to make decorations, including Christmas wreaths, and when it falls to the ground, it nourishes the soil with nitrogen. If you just don't like it, and want it down, high winds will sometimes blow it off, long-handled pruners can knock it off, and dead limbs. when trimmed wlll often bring some with them. Then all you have to do is clean up the mess.

Lichens are an even more complex subject; they are usually composed of at least two different organisms and can also function as epiphytes on tree branches. They don't appear in our Native Plant Database because our expertise is in vascular plants native to North America. However, we found an excellent  website The Special Biology of Lichens, that will probably tell you more than you ever wanted to know.

Conclusion on comparisons of harm done by ball moss and lichens? No contest, there isn't any harm done  by either.