First, no matter what form or color your lichens are they are harmless to the affected plants.
I found it said that the use of copper-based fungicides are recommended for lichen control. These fungicides are sold under many different trade names.
Such products contain copper hydroxide.
Mix together 1 level tbsp. of chemical in a gallon of water and spray on affected plant areas.
Repeat applications every one to two weeks.
But at the end of the day unless you want to, you do not need to do anything.
However as an FYI:
Lichens are most attracted to plants with declining and poor health. Because of that: Do not overly prune plants as too much sunshine can promote lichen growth on Crepe Myrtles.