Your trees appear to have some resurrection ferns and lichens, both of which are generally considered to be harmless to the tree.  Resurrection ferns are air ferns and like Spanish moss this epiphyte, or “air plant”, uses a host plant (live oak, pecan, or sabal palm) as its growing surface. Yet the fern does not damage the tree. Its root system is composed of long, winding rhizomes that tuck into the nooks and crannies of the tree’s bark, collecting rainwater and nutrients from airborne dust and nutrients leaching from the trees themselves.  Lichens are an example of a symbiotic relationship between algae and certain fungi. They are capable of producing their own food. The algae associated with the fungus is a green or blue-green alga. There are three forms of lichens based on growth patterns. Crustose are species that are closely pressed against the surface of the limb or trunk of dead or live trees. Foliose forms are leaf like or prostrate but are also tightly attached to the tree. Fruticose forms are bush like, erect or hanging. Although lichens are found in most areas of Texas, they are most noticeable in areas that have extended periods of high humidity.  Presence of lichens also is a good indicator of a thin tree canopy. This often leads homeowners to conclude that lichens are the cause and not the effect of thin foliage. The best control for lichens is maintain the tree in good condition. This will insure a dense canopy which will shade the limbs and reduce photosynthesis. Without photosynthesis, lichens and resurrection ferns are not able to manufacture food needed for growth and development.  The best thing to do is fertilize the tree to increase the overall health of the tree. Fertilize with 10-10-10 fertilizer at the rate of 2 lbs per inch of trunk diameter scattered around the tree and watered in good . Apply just before a rain storm and you will not have to water.  Well fertilized, healthy trees should reduce the population you currently have in your trees.