There is no treatment, it’s a fungus that is being fed by the wood. It’s supposed to go away with time. You can get otto’s or algae eating shrimp to help it disappear faster. However, there is no chemical solution. If you keep scrubbing it will continue to come back so let it run it’s course. It’s ugly as hell I feel your pain. I’m just starting to get rid of mine and it’s been a month +