I learned from a previous same situation that there is a product sold in the home centers that will assist in removing the old film.
What you must be careful about is damaging the glazing on the good tiles while trying to remove the grout.
I see this happening so often on this site. When doing this again, be sure to start removing the excess grout and film no more than 20 minutes after applying the grout. If so, it comes off easily, but with many rinsings.