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.
Clean the floor sweep or vacuum the floor thoroughly then wet mop the tile to dampen it.
Protect your hands with rubber gloves and dip your cheesecloth or terry cloth in water.
Remove grout haze with water and cheesecloth.
Since nothing you tried removed the grout haze i d guess that the slate probably wasn t properly sealed before the grout was applied.
Wring thoroughly as too much water can damage.
Scrub the surface of the slate tile with a generous amount of water and a nylon scrub sponge.
Instructions wait for the grout to cure wait at least 10 days for newly installed grout to cure before using grout haze remover.
Pick up the dirty water with a clean grout sponge.
How to remove grout haze from a slate floor 1.