If you wash the whole slab with mild soap (like dawn) what happens? If the stain fades while it’s wet and then re appears an enhancing sealer could fix it. Unrequested opinion but when you can replace the quarter round around the hearth with wood stained to match the floor or painted black to match the hearth- the white is distracting.

When washed, it does temporarily disappear, then reappears once the surface is dry again. I completely agree about the quarter round. It was put there by the flooring installers without my feedback on it. Thank you for your reply!

Try heavy duty magic eraser. If no improvement, advance to sandpaper, starting at the finest grit.