I have a tiled shower stall that is probably 30 years old or so. I had a few small spaces where the grout had come out that I was caulking and discovered that the grout is very soft. If I scrape it with my fingernail it comes off, sort of a powdery paste consistency. Aside from this grout issue the stall is in very good shape with no evidence of leaking (it is a ranch and there is no evidence of leaking at the exposed flooring and joists seen in the basement under the shower. The shower pan is not tiled, it is a one piece, maybe acrylic, pan. The grout issue only affects the walls.
What is going on with this grout? The grout that is outside of the shower stall seems fine. More importantly, what do I need to do to fix it? Do I need to remove all the grout in the stall and re-grout?