Fine dirt or black algae???

Having trouble keeping pool clean this season. We have a 20 x 40 kidney shape vinyl liner in ground pool. Triton II sand filter. Where the seams are and any small dips in the floor, there seems to be dark dust like build up. You can brush it away turn the pump off for the night. In the morning it is still clear. When I turn the pump on for the day, within an hour there is more dark scattered at seams and on the floor. I thought that the filter might not be doing its job. Because you cannot pick anything up in the net or the Polaris I am thinking it might be algae. I have read some of the threads about black algae, but not sure about vinyl liner pools. I have had a metal problem too. It seems to be better, but if I have to put a mega dose of shock it may turn the stairs orange....water chemistry quite complicated...fix one thing mess something else up:/ any suggestions on how to treat?