Answer: Gray green patches sound like lichen and it won't hurt your tree, but it does look unsightly. You can spray with lime/sulfur in the wintertime to kill it off and it will slough off on its own. That's not what is stunting the growth of your oak, though. It's more likely a problem with the roots - they may have hit hard pan or they may simply need more water than you are supplying. Try building a watering basin beneath the tree and flooding it, allow it to drain, then flood a second time. This will concentrate the moisture over the root system and allow it to trickle down and wet the entire root mass. Best wishes with your oak tree.
Q&A Library Searching Tips