I'm a newbie to this forum. I don't have a pool but my sister does and I am trying to help her out. She has large rocks/stones piled up in the back of the pool with a waterfall. There is a scaly, rough, white material that has built up on the pool tile directly under the stones that appears to be wash out from the material holding the stones together. She has tried many different products like acids and CLR but nothing has helped. Can anyone recommend something that could remove this?