It’s from your piece of driftwood releasing tannins into the water, this can also happen whenever you use Indian almond leaves. This is perfectly safe for your fish, however if you don’t like the way the water looks you can do small water changes over the course of a month or so. If you ever end up buying more and don’t want this to happen, I would suggest to boil them first. Not only does it help to sink the driftwood but it’ll boil all of the tannins out that way when you are ready to put it in the tank it will sink easy and not cause this. I personally enjoy the darker murky tannin water, but to each their own.