If there's a trash can or a planter or a bench, see if you can move one on top of the spot
Really, that mineral discoloration or whatever it is much harder than concrete, like granite its just permanent. Have you poured acid on it and seen nothing happen? If you put Klenstone or Glazed surface restorer (eaco chem) and cover with plastic, then continue to add a little at a time while keeping covered, I bet that would eat through it. But it won't look great and is likely to come back quickly. Both those products have sodium bifluoride, which I think turns into hydrofluoric acid when you use it?
Unfortunately if there is a depression in the concrete it will continue to have water puddle there, creating the stains from the hard water we experience here in Arizona. Try Rust a Way or Iron Out, might remove the stain, but like I stated the stain will return once water puddles there again.
Our water here is rediculously hard. When someone really wants that stain off, we have to us hydrofluoric acid. We spray on the cleaner, let it sit and then grind at it with a floor machine equipped with an aggressive brush.