Cement grout, epoxy grout and flexible sealant are different materials. Repeated acid and force can remove the joint without solving the moisture, residue or installation problem beneath the stain.
Cloudy shower glass may be wet condensation, soap film, mineral scale, scratches or chemical corrosion. A small dry test can separate removable residue from permanent damage before stronger cleaning makes the problem worse.
Limescale can yield to acid, but stone, plated metal, protective coatings and appliances do not share one safe recipe. Identify the deposit and finish before choosing the method.