Glad you checked, because white, green and yellow all have different diagnosis, and as you can see, it's really hard to tell looking at the tank.
Like BigJim said, it's most likely tannins leaching from the new wood. It's completely harmless, so nothing to be worried about. If you don't like the way it looks, however, there are things you can do to stop it.
Again, like Jim said, running AC will help to remove it. If it really bugs you, you can take the driftwood out and boil it in water until it stops staining the water. Depending on the size of the wood, this can take minutes, or hours to do, however.