Surface staining in grout is almost always ordinary mildew and cleans off with a bleach solution and a stiff brush. If it returns within a fortnight in the same spot, the grout has failed and water is getting behind it — at that point cleaning is treating a symptom.
Clean it once, and pay attention to what happens next
One part household bleach to three parts water, a stiff nylon brush, ten minutes of contact time, rinse well, and run the fan. Do not mix bleach with any other cleaner — particularly not anything containing ammonia. Open the window.
The diagnostic is what happens over the next two weeks. Mildew that stays gone was a ventilation and moisture problem. Mildew that comes back in exactly the same line or corner is telling you the grout in that spot is porous or cracked and is holding water inside itself.
Grout is not waterproof, and was never meant to be
This surprises people. Grout is a filler; the waterproofing is the membrane behind the tile. In a properly built shower, water that gets through the grout hits that membrane and drains back out. In a shower built before waterproof membranes were standard — or built badly — it hits the substrate instead, and the substrate rots.
That is why recurring black staining along the bottom row of tile or in the corners matters more than the same staining on a wall halfway up. The bottom row is where water sits.
The corner test
Corners should be caulked, not grouted. A tiled corner moves — two planes meeting at ninety degrees expand and contract differently — and rigid grout in that joint cracks every time. If you have grout in your corners and it is cracked, rake it out and caulk it with a proper sanded silicone. That is a Saturday and about thirty dollars.
When it is actually a remodel
Press on the tile around the recurring stain. If it flexes, sounds hollow, or the grout crumbles under a screwdriver, the substrate behind it has gone soft. No amount of cleaning or re-grouting fixes that, and patching over it just hides the rot while it spreads. That is the point where a tear-out is the cheaper option, because the longer it sits the more of the wall framing you end up replacing.
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