Who is the tile manufacture?

It's moisture behind the tile which is creating mold.

The surface was not properly prepped with among other things, a waterproofing agent.

It will more than likely be an ongoing issue.

Same assumption like Mystic Pools.

But do all pool tiles have the same colour? It looks like it´s only a problem withe the dark gray ones. Maybe that's how it should be and they change the colour with the years.