According to the spruce.com...
Soap scum is formed into a solid substance when soap is being used in hard water. On a scientific level, soap scum combines calcium and magnesium particles (ions) that are currently in the water with the soap, which forms into the soap scum substance. This formation is frequently caused by minerals in tap water that combine with soap and dirt to create a layer of scaliness over the surfaces in our bathrooms and sometimes even our laundry. Those with mineral-filled hard water are much more likely to have soap scum buildup, which can be difficult to remove if left for too long, as soap scum continues to build up each time the area around it is used. Soap scum that is left alone for build-up can combine with mold or mildew and have other discolorations and odors—and can even get as hard as concrete.
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