You should moisten the surface of the tile with cold water, and then sprinkle the stained areas with salt, which will corrode the cement after some time.
You can remove the remaining stain using a regular stiff brush.
The cement mixture is soaked with ordinary water, and the polymer adhesive is exposed to weak acids or solvents.
Lemon juice or vinegar will do, as will chemicals to remove stains in the bathroom.
The softened layer loosens and is easier to remove with sandpaper or pumice.
The remaining solution can be easily removed by soaking it with acid.
To do this, soak a sponge or piece of foam rubber in hydrochloric acid, place it on the underside of the tile and leave it like that for a couple of days.
In order to wipe off the remaining dried paste, abrasives are used.
The simplest one is table salt. This is a gentle product that can be used on tiles or glass without worrying about scratching.
Harder abrasives are steel wool and pumice.