I just had my concrete pool deck acid-washed. One of the workers walked through the house and stepped on my marble floor, leaving footprints which won’t come off. What can I use to remove them?

The acid has etched your marble, which will now have to be polished or (possibly) honed. Depending on how deep the marble is etched it may require a heavy grinding. Perform a test in a small area to determine what grit diamond will be required. Make sure you test it throughly. Hydrochloric acid is commonly used for cleaning concrete and can be very destructive to marble. So be sure to hone the floor throughly. It is also a good idea to rinse the floor before starting with ammonia and water. Ammonia will neutralize any residual acid.