Efflorescence is common on many exposed concrete and cement mortar applications. Generally it is seen when cracks in concrete or mortar are exposed to water rather than accumulating within the crack. The basic mechanism creating efflorescence is when concrete is exposed to water for a long time; excess free lime (calcium hydroxide) in the cement paste goes into solution with water (leaches). Then when that water eventually leaves the crack and dries on the surface, the white residue of calcium hydroxide creates what is termed “efflorescence.”
It is very common to see efflorescence on brick structures where the mortar joints are exposed to rainwater that leaches out the calcium hydroxide and the resulting white efflorescence is highlighted on the dark-colored face of the brick. In concrete tanks, it is often found in cracks that can accumulate water for a sufficient time to leach the calcium hydroxide. The bottoms of vertical cracks or low areas in horizontally oriented cracks often show the greatest buildup of efflorescence. These can be surface cracks that are exposed to rainwater or through wall cracks that are exposed to water contained within the tank.
Although the tank was cured properly to help deal with long-term drying shrinkage, surface cracking on shotcrete often results from early-age plastic shrinkage cracks. These are shallow cracks that form within hours (or minutes, in extreme conditions) of placement due to rapid evaporation of water from the exposed surface of fresh concrete (common in exposed floor slabs or in your case the fresh shotcrete wall surface).
To answer your question regarding when it will stop, the answer is it won’t unless the cracks are sealed, or water is prevented from getting into the cracks. Cement-rich shotcrete has more than enough free lime to continue the leaching for decades. Although surface-applied coatings may initially span small cracks, as the walls of tanks expand and contract due to filling and emptying, and undergoing daily and seasonal thermal changes, the surface cracks will open and close slightly and eventually mirror through the coating. Coatings designed to tolerate moving cracks would likely be much thicker than the 7 mils used on your project. If the cracks are through-wall cracks that are seeping from the contained water, the crack will need to be sealed, most commonly by injection of polyurethane grout or interior surface coatings.
To answer your question on how to prevent this in the future, early-age plastic shrinkage cracks can be reduced by fogging the fresh shotcrete surface to keep the surface humidity high and reduce evaporation of the water at the surface of the concrete. Also, using fibers in the shotcrete can help reduce plastic shrinkage cracking. In hot or windy climates, placing the final layer of shotcrete during the coolest or calmest time of the day may help, too.
To answer the question if additional paint would seal the cracks, simply coating with an additional 7 mil (0.2 mm) coating would provide a temporary seal, but more than likely the crack will mirror through after some period of exposure. A coating designer would need to evaluate the crack widths and potential movement to design a coating system that would provide a long-term seal.
Finally, the efflorescence caused by exposure to rainwater is generally only a visual defect and doesn’t affect the long-term structural integrity or durability of the tank. Many owners tolerate efflorescence on the tanks and simply clean it off when it becomes objectionable.