The guy has used the wrong grout on the walls, the yellow bagged grout from palace is their wide joint floor grout, its a much courser sandier grout than wall grout, maybe its reacted a bit more with the drying process of the tub gear on the walls and not the cement adhesive that would have been used on the floor, Palace only do white wall grout.
Check out the palace chemicals web site print it off about grouts and tell the pro
tiler to change it to a proper one.
Maybe hes not such a pro after all