Tile – What are some methods to get rid of and prevent pink mold in the bathroom

My experience is that the black growth actually gets right into the bath mat material and cannot be easily removed. I suggest that you replace the mat and then use an alternate procedure when stowing the mat after use.

After use always rinse it off well so no bath scum and other things are left on it. Then hang it up over a towel bar, suction cut side up, so that it can fully dry after each use. When you suction cup the wet mat to the wall you are leaving it there as a perfect breeding ground for mold and mildew growth to occur.

Since they're up, I agree adding blocking is a PITA.

However, if its an inside wall that you have access to the other side of, use a "BIG GAP" polyurethane foam. Drill a series of 1/4" holes every 12" down the middle of each joist bay. Spray a 3 sec burst in each. Wait 12 hr and try the flex, it should be quite firm now.

You may not want to do this near the plumbing valve, for replacement purposes. It won't hurt it, though.