I use the BBB method and my water always looks great and my chlorine levels are good but the walls on a little over half my poo(24' round) is always covers with brown stuff. I'm assuming it's dirt and grime and not algae but I don't know any more. It is all over the wall in the direction opposite whichever the return eyeball is pointing. If I brush the walls it comes off and goes into a cloud of circulation until it filters out or settles. I keep my free chlorine between 2-6 and never have any cc on the test (never turns back to pink). So is this normal? Is it dirt or possibly algae? How do I fix it?