That looks like lichen and moss, non-parasitic organisms that colonize the bark of trees. They do not harm the trees they are growing on. It is common for algae, lichens and moss to be found on trees. They tend to be found in damp humid places, and lichen in particular is commonly found on trees that lack vigor (i.e. slow growing).

You don’t need to do anything to control it, but you can make the conditions less suitable for their growth. Water at the base of the tree. Don’t use sprinklers or other irrigation that may hit the bark or canopy. You can reduce humidity and increase light by removing weeds and other unwanted plants around the base of the trees, and pruning back neighboring plants to improve air circulation, or pruning the effected trees to open up the canopy to more light and ventilation. For more information, refer to https://ipm.ucanr.edu/PMG/GARDEN/ENVIRON/lichens.html

The one thing that I couldn’t get a good look at, are the black spots on the trunk that I see in the first photo. Those spots are likely a lichen or possibly sooty mold that grows on sap. The spots don’t look sunken, which may be an indicator of a developing canker, but keep an eye on these spots to see if they develop or change form. If you see other symptoms on the tree, such as wilted leaves or branches that dieback, then send some photos and I can take another look at it.