For less-futile scrubbing, the absolute best cleaning device I have ever used on soap scum on glass is the tangled clumps of hair I pull out of my hairbrush. (I discovered this because I used to brush my hair in the shower before it got too long to do that without using a half-ton of conditioner every time.) On glass, it compares favorably to green scrubby pads; I'd say at least twice the cleaning power per effort. Though a wad of hair pulled out of the hairbrush is not the most squick-safe item in the world.