Efflorescence

The concrete shell of your pool can act like a wick, sucking up groundwater and then oozing out through your grout. As the water travels through your concrete, it becomes highly mineralized, increasing the pH. When it exits through the grout, it evaporates and leaves all that scale behind.

Your pool is a giant concrete shell buried in the ground. Concrete is NOT watertight. So think of it like an unglazed clay pot submerged in a sink full of water (your ground is saturated with water right now) - the clay pot will absorb water.^[1]

Your concrete shell is like a clay pot: It constantly absorbs water. Due to its porosity and capillary forces, water moves through the shell, looking for any exit point it can.

The plaster coating your pool surface is hydraulically sealed so no water can penetrate the plaster. Your ceramic tile is hydraulically sealed as well.

On the other hand, the grout between the tiles is not hydraulically sealed. So, as the water moves through your shell, it looks for an exit point, and your grout is the perfect spot for it.

Many pools are constructed with a water barrier material coated behind the tile to reduce water penetration into the shell. That coating can get compromised over time, and some pool builders neglect to coat the waterline and all spa surfaces properly. Spillways are often overlooked.

Without a barrier layer, water moving through the shell will go through the grout. The water absorbed by the concrete and coming out through the grout absorbs calcium from the cement and increases pH.

When it hits the air, it evaporates and leaves behind all those minerals and scale it absorbed. This is how efflorescence works. It's quite common in cement structures, and you see it all the time in concrete bridges near bodies of water.

When it hydrates and hardens, Portland cement always produces Calcium Hydroxide (CH), which can be a significant source of efflorescence. Pool plaster and all concrete are porous, and it is easy for soluble salts to migrate through the material. The concrete industry refers to primary and secondary efflorescence. Primary comes from the material itself (ie, the CH in the pool plaster), and secondary comes from the underlying substrate, the earth around the pool. For the salts to move through the material there has to be a moisture drive. This is why it typically shows up above the water line - evaporation of water from the dry surface deposits salt on the surface. The moisture could come from the surrounding earth or from the pool water itself.