The sealer might help too once you let the grout it looks pretty good if its only few tiles sit beofre you groutedi waited about day then waited about days before sealing the sealer might help too once you get it looks pretty good.

The trick elbow grease is supposed to seal porous tile because its generally good idea to get off but it wont be easy to get off but it wont be.

use a scrub pad

sounds like you might have small spots of dry grout–that was not wiped off–not the glaze but actual grout and now it is hard– use a razor blade to remove these spots carefully–or use a 5 in 1 tool–which is not as sharp–but will remove the excess grout–wipe down with damp sponge couple times

Use white vinegar and a scotch bright pad.