sand covered in purple gunk WHY?

[Josh_22]

my white sand has always been on the brown side so when I was at the LFS i ask what I could do to whiten the sand and they told me to buy a white sand sifting star. And I did. He was right the star cleaned the sand and it was whiter then I had ever seen. Then the star moved to the back of the tank and all the sand in the front of the tank turned purple. way worse then it was before.
Why did this happen. I have a 3 to 4 inch DSB in my tank.
Thanks
JOSH