That moisture is coming from behind the tiled wall.
Grout or caulk between wall tile and tub.
Caulk is a sealant typically made of silicone or acrylic material.
And then there s this from a home inspector.
Or where the edge of a tile meets the wall.
Then once it gets wet for a while the caulk basically turns to gooo and can be wiped away.
Caulk seals seams between tile and fixtures or walls.
After caulking the tub tile joint with a bathroom silicon and letting it sit for at least 2 days empty tub it all seems to have hardened and looks good.
Caulk should be used between the tile planes where there is flex and movement between the tile planes.
Caulk traps moisture that s behind it by not allowing it to evaporate.
A carefully run line of caulk known as a.
If this happens in your home remove the grout and replace it with caulk.
Caulk is impermeable meaning it won t let in water.
Let the area dry the area for the minimum of what the moldicide recommends which is usually at least 24 hours before applying new caulk.
Caulk provides a waterproof seal where tile abuts other materials such as bathtubs sinks or walls.
His explanation makes sense.
As a home settles however grout can crack.
Hence the puzzlement of why one can t clean caulk that appears to be sound.
Grout not caulk where tile meets tub.
Some contractors grout the space between the tile and tub.