Greetings, welcome to the Forums.
What a beautiful orange lichen, both lignicolous and foliose. It may be Xanthornina parietina, or something similar.
Lichens are a diverse group of fungi that live on various substrates in symbiosis with photosynthetic algae living within their tissue.
Lichens are not parasitic to the trees on which they grow.
Many lichen are sensitive to air pollution, and a survey of the types of lichen able to grow at a given site may be used to estimate pollution levels. How wonderful that your air is clean enough to have these intriguing organisms in your garden.