Rusty type stain on my steps-( pic inside)

Have this rusty type stain upon opening of pool. tried the magic spong which got 75% off but some remains-. any ideas?? on what it is and how to remove it??

thanks-


[IMG]http://i3.photobucket.com/albums/y78...psbxdlj6hl.jpg[/IMG]

Could it be an iron stain? Do you fill from a well?

Thanks no I do not fill from a well. however iron was my thought. anyway to remove it...