from what you've said, it's possible it's tannins from the driftwood, some wood can still leak them after a year or more. the charcoal would filter it out, when the charcoal filled up you would see the tannins build up again.
pull out the wood and keep up on the charcoal replacement and it will probably clear up.
tannins are no problem for the fish, and in many cases are beneficial. I use indian almond leaves in my blackwater tanks to keep the water that golden brown color.