Looks like a lichen
I moved some flower pots on the porch the other day, and this little tree frog leaped out of them and stuck himself to the side of the house.
I've seen frogs of this sort before, but usually not on a tree. On a tree, their grayish coloration hides them very well. They look like lichens, and most of the trees out in the country have lots of lichens on their trunks. The tree frogs I spot, like this one, are somewhere that their camouflage doesn't work.
While I was looking at this little frog and taking his picture, he didn't even twitch. I suppose that's his instinctive behavior. If he sits completely still, he looks even more like he's part of the tree.
When I came back to the area a few minutes later, he was gone. He knew I had stopped hovering over him, and he seized the opportunity to get away.
I hope this little fellow found a warm burrow, last night. There's frost on the roof of my car this morning.