I just completed polishing a marble floor and the grout is covered with streaks and spots. I am sure it is not polishing powder. How can I repair the grout?

Short of removing it and installing new grout I would try cleaning it with stone soap or neutral cleaner. Be sure to use a soft brush and scrub the grout lines throughly. If this does not throughly clean the grout you may need to replace it.

TIP The spotting you describe may only be moisture in the grout. Before attempting any repair, allow the grout to dry for several days. If the spotting remains after this time then try cleaning.