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.
Article ID: 542
Created: January 20, 2022
Last Updated: January 20, 2022
Author: Pam Freitag
Online URL: https://sr-manual.com/kb/article/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-542.html