While it's been a week, there's no way for us to evaluate the adjacent soil moisture condition from afar, if that's what it is. Regardless, it is not a eye sore type stain and relatively minor to the overall appearance. Since it's new concrete, it has not begun the aging out process. Just look the other way and this will resolve itself in time. Usually, most attempts to fix a "problem" like this -- with acids or chemicals strong enough to have some effect on concrete -- make the "problem" worse. Sometimes MUCH worse. I would not do anything to it.