What is a brown sandlike substance that is on the bottom of the pool - even after vacuuming - it returns within an hour or so - we have tried algae killing chemicals and everything - it doesn't help?

The sand could be sand from your filter. If you have aDE filter the screen could be misaligned. If you look and see if any water is coming out the backwash port while in filter mode the valve is bypassing. Does the pressure build up when you don't backwash for awhile? You might have a lateral broken inside the filter.