I HAVE WHITE STUFF ON MY WALLS THAT LOOKS LIKE MOLD.

“Efflorescence is a deposit of mineral salts on surfaces of concrete and masonry walls and/or floors.  It is whitish in appearance, and is sometimes referred to as “whiskers”.

https://wet-seal.com.au/efflorescence-causes-removing-prevention/

Here in Mexico it is called salitre and by the time it becomes visible, it means that maintenance is required. The word ‘sal’ means salt and has led to myths of people saying that beach sand was used in the construction, thus leading to ‘salt’ in the walls. In reality, we find salitre in areas that have no leaks but are simply extremely damp. The dampness either from a closed off room, water leak, or the combination of mineral salts used in construction materials, results in a perfect storm.

In the end, what is important is to be able to identify salitre as soon as possible. The longer salitre sits, the more it destroys and the more difficult it is to make repairs unnoticeable. In some cases, we apply chemicals 3 days in a row and on the 4th day we repaint. If the salitre has eaten away at the plaster, we need to remove all loose plaster and apply the 3-day treatment, re-plaster and paint. In many homes one can see where there have been salitre problems simply from the replastering. Treating salitre is quite common in our line of work but it doesn’t mean that it gets rid of it. Sometimes the salitre moves down the wall or floor. Other times it stops completely and sometimes it just waits a few months to say hi again. In either case, a good property management team knows how to handle salitre.