I'm on my phone and can't enlarge the pic enough to see the stuff clearly. However, I have several older azaleas and rhodies, and they all have lichen. The lichen mine have is sort of flaky looking stuff. The guy at the rhodie and azalea nursery told me its common as the plants age, and recommended I not do anything about it.
My plants had it when I bought this house back in 1986, and they're still strong and healthy and bloom well every year.