We had a similar situation with a floor installation. On a vertical surface, this would obviously be more challenging, with limited dwell time.
The installer applied grout sealer, which appears to have bled into the stone. To
resolve the issue, we applied an alkaline cleaning product (Modern
Clean by Modern Stone Technologies) to the floor, scrubbed with a
tampico brush on a floor machine, & extracted with a truck mount. I've attached a before and after picture. If you look closely at the after, you'll still see some residual discoloring, but overall it worked well.