Efflorescence
The concrete shell of your pool can act like a wick, sucking up ground water, and then oozing out through your grout. As the water travels through your concrete is becomes highly mineralized and the pH goes way up. 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 water tight. 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 the clay pot, it is absorbing water all the time. Due to it's porosity and capillary forces, the water will move 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.
The grout in between the tile, on the other hand, is not hydraulically sealed. So, as the water moves through your shell looking for an exit point, your grout is the perfect spot for it.
Now a lot of pools are constructed with a water barrier material coated behind the tile to reduce the penetration of water into the shell. That coating can get compromised over time and some pool builders neglect to do a good job of coating the waterline and all spa surfaces. Spillways are often neglected.
Without a barrier layer, water moving through the shell will make it out through the grout. The water that gets absorbed by the concrete and comes out through the grout absorbs calcium from the cement as well as increases in pH.
When it hits the air, it evaporates and leaves behind all those minerals and scale it absorbed along the way. This is how efflorescence works. It's quite common in cement structures and you see it all the time in concrete bridges that are near bodies of water.
