Brown dirt/dust on bottom of pool

[tschmidt]

Hello. I have an above ground sand filter pool going on 17 years now and never had this problem. After I run the filter over night there are brown dirt/dust spots on bottom of pool. If I dont run the filter all day then there are no spots and its crystal clear. I had the pool store test the water and they said it looked good but to add a little alkalinity up to it. I vacuum it up with the vacuum robot and I also used the hose attached to the filter and they go away until I run the filer again. Any idea's. Thanks