Howdy, and thank you for your inquiry. There are a few organisms that live on the surface of tree bark. One is often mistaken for a fungus and is called lichen. Lichen are made up of a symbiotic relationship between fungi and green algae or cyanobacteria. They are not harmful to the host tree, and more information from an article from the Texas Plant Disease Diagnostic Lab's website for comparison: https://plantclinic.tamu.edu/2019/02/22/you-either-lichen-or-you-do-not-like-em/; additional images from another AgriLife article online: https://plantclinic.tamu.edu/2014/04/29/are-lichens-killing-your-trees/

If you are requesting risk assessment to determine the structural integrity of the tree, it is recommended to contact a certified arborist. 

Browse online to the International Society of Arboriculture: https://www.treesaregood.org/findanarborist . Choose the image "Find an Arborist". You will be forwarded to a webpage to search by country (USA), then forwarded to a page to further refine your search by choosing your postal code. I recommend to place your search within a 25 or 50 mile radius. The programmed search engine will then shuttle you to a page with a list of arborists and their credentials. Most of the companies or individuals are located within or near Houston, an advantage regarding proximity.

Let me know if this information is useful for you, and the progress of arborist consultation. Contact me directly by email if you need further assistance: <personal data hidden>. I am closing this online correspondence.