Roger,

 

I am assuming this is from regular grout and not epoxy. If so, that an mild acid may do the trick. Since the stone is already etched you will need to polish it anyway. I would try using a solution of sulfamic acid. Mix according to directions and agitate and than rinse.

 

Now, if it is epoxy grout  I would go the shoe store and buy yourself a new pair of running shoes and run away as fast as you can.  Really, expoxy grout stains can be nearly impossible to remove.