I have a gunite inground pool with about 21,000 gallons of water with a Hayward DE filter. I just opened it last Sunday and did my usual shock, algecide, and vacuum routine. There is about 4 inches of dirt stains right below where the tiles stop all around the pool. I tried using a brush on it, but it does not come up. What can I do to get rid of it? Superchlorinate it? Thank you.