Is the floor previously painted, besides the overspray? Mine was coated with a cheap epoxy paint when new. I get overspray on it all the time. Most of the time a good scrubbing with laundry soap will get MOST of it off. The rest wears off pretty quickly. Of course, spilled paint is a different story!
That'll give the new floor coating something to bite into!! Other than scrubbing or a floor polisher / sander you might pretty well be screwed. I would let it dry and maybe rent a floor scrubber with the scotch-brite pad and go over it using some TSP to see what that will do.
The natural concrete lets the paint bite into it beings that the sealer has probably wore off in that amount of time.