Lichen is not an indicator of health-- nor do they harm the tree. The actually don't absorb nutrients at all from the surface they grow on. The are an indicator of good air quality-- though most of us don't live urban enough to worry about that too much, I think.
Lichens of this type indicate two things: The air is clean and that area is getting a lot of sunshine.
People think that lichens only attach to unhealthy trees-- not so. It attaches to all things, as you have observed. It only seems to grow in great abundance on live wood that either: gets a lot of sun (such as broad horizontal branches of large mature trees), or if the branch is already in decline, reducing the amount of foliage and therefore allowing the lichen to get more sun.
I know this does not necessarily help you but lichens are often misunderstood.
By the way, this looks like perhaps Wax Paper Lichen, or Hammered Shield Lichen.