This is a picture of what appears to be a healthy growth of Xanthoria parietina more commonly know as Yellow Scale or the Common Orange Lichen.
Interestingly lichens are not plants but actually both fungi and algae living together in a symbiotic partnership.
The growth seen in this image is over the branch of a tree but lichens are not parasitic and are equally at home growing on stone. They use photosynthesis to generate their own food and use what they grow on as merely a substrate upon which to anchor.
Lichens are known to be susceptible to pollution and are a good indicator of air quality so if you find yourself somewhere with an abundance of lichen be sure to take a deep breath.
Don’t breath too deep if you see this particular lichen though, it is known to be pollution tolerant which is probably why it’s so common.