Last week we cleaned limestone walls and floors for the residential customer  using our truck mount. We came back yesterday to seal the floor and were surprised by the look of one slab, with one continues stain at the bottom.  It lookes like iron oxidation and it is only 1 slab out of 6. We tried iron out but that didn't help. 
Any idea why something like that happened?
What can I use or do to remove it?
I am going back there on Thursday and I need your  help very badly. I am puzzled and without any ideas. I am attaching picture for u to see it.
Thanks
Adam Bartos
BiO Tile