pool pump problem

[Tammy]

My pump is running on low and water is clear turn it on high and it blows out a film of white dust seems like.....I am constantly back washing to get the dust out.....I think it is shock residue. Do I need to add more sand to my filter. I replaced the sand last season never really measured just tried to put 75 pounds of sand from 2 - 50 pound bags. The filter is running at about 12 pounds of pressure at high speed, but residue keeps building up and coming back in the pool. Please help!