Normally a scotch brite pad water and a little rubbing will remove grout residue and the hazy appearance it causes.
Cleaning grout residue from slate tile.
Wring thoroughly as too much water can damage.
Instructions wait for the grout to cure wait at least 10 days for newly installed grout to cure before using grout haze remover.
How to remove grout haze from a slate floor 1.
Since nothing you tried removed the grout haze i d guess that the slate probably wasn t properly sealed before the grout was applied.
Protect your hands with rubber gloves and dip your cheesecloth or terry cloth in water.
Remove grout haze with water and cheesecloth.
Scrub the surface of the slate tile with a generous amount of water and a nylon scrub sponge.
Use another clean sponge to rinse the scrubbed slate tiles with.
Work on three or four.