Low CSI might contribute to spot etching, but I don't think that it is the primary cause. Spot etching is where there are small spots of damaged, discolored or etched plaster where the plaster is noticeably different in color, composition, porosity or texture.
I think that the term is somewhat controversial and might be caused by several different things. Some definitions don't include actual etching or texture differences. The increased porosity can allow algae to dig into the plaster. The porous spots also will tend to absorb metals, and you will see metal staining in those areas.
I don't think that aggressive water alone causes spot etching, because if it were just the water, then the etching would be more uniform. There has to be some sort of defect in the materials or application to cause spot etching. I think that the spots have to be weaker than the surrounding plaster and are therefore more vulnerable to aggressive water. Due to the defect, it is likely that the spots would become etched even with balanced water.
Spot leaching might be a better term. I think that aggressive water accelerates the etching (or leaching). I think that some of the causes could be the following:
1) Excess calcium chloride used to accelerate the cure rate of the plaster.
2) Calcium chloride being added in a way that leaves lumps of calcium chloride instead of fully dissolved and distributed calcium chloride
3) Lumps in the cement or aggregate that do not get fully mixed and hydrated.
4) Workers throwing or spraying water on plaster that is setting up to make it more workable. Where the water hits might cause those spots to be weaker than the surrounding plaster (possibly by washing out the cement and leaving the aggregate). Workers are more likely to need to use water to rewet the plaster when excess calcium chloride is used and the plaster sets up too fast.
For plaster to last as long as possible, it is important to keep the water fully saturated with calcium carbonate. I always keep the CSI in plaster pools on the 0.0 to +0.3 range, I keep the pH in the 7.7 to 8.0 range, and I do not get scaling. I have pools with the same plaster for 20 years and the plaster is still in good shape.