I end up letting it go for so long that I'm surprised by the difference. I probably let it go for about 8 months this time.
I use those disposable straight razor blades with the plastic grip. They're kind of small for the job, but I can get most of the stuff off with one pass (which is important when I get my clumsy hands too close to the SPS).
The overflow is black acrylic and the coralline lifts right off of it with the razor. Probably easier than the glass. Except for the seams, which is why my tank always has an awesome purple stripe in the middle. If I were to have this overflow box built over, I'd request for that seam be on the bottom of the box.