Have you noticed a white residue on your grout that keeps coming back? It's likely a result of a natural occurrence called Efflorescence.
Efflorescence is a natural occurrence in Portland cement applications, such as tile adhesive. What happens is that the water (common in bathrooms) travels through a water path (crack) in the grout that is in contact with calcium salts which are in cement-based products, and then is drawn out to the surface through the pores of the grout. Once the water dries it leaves behind a white residue.
The white residue comes about from leached lime exposed to the atmosphere, then reacts with carbon dioxide to form calcium hydroxide crystals. It can be on the grout joints or even over the edges of the tiles.
Efflorescence is harmless, but can be quite noticeable particularly with darker grout colours. It can occur on both wall and floor surfaces. It can be cleaned off but likely will continue to occur unless the problem is removed.
You may need to seek professional help for this. The best method is to try and stop water from getting behind the tiles, so fixing any cracks, including hairline cracks in the grout that may have occured with a lot of movement.
There are products on the market today that inhibit effloresence or even have zero effloresence.