Why would this help

Because the problem right now is too much of the Alkalinity component in the water. Decreasing salinity decreases Alkalinity. Alkalinity is carbonate ions floating around in the water waiting to combine with Calcium. Removing some salt water and replacing it with pure water will dilute the amount of those ions, making the water more bearable for the aquarium inhabitants.

But won't decreasing the salinity hurt the coral

Actually no, not if it's done over the course of an hour or so. In the wild, these animals are genetically tuned to having salinity go down quickly, like when a torrential rain adds millions of gallons of fresh water to the reef and tide pools. Ask anyone that has seen their top off water do a small flood of their aquarium. Salinity can drop fairly rapidly to 1.022 or even 1.020 without any bad effect on the animals. But when raising the salinity back up, do it more gradually because there is no genetic variable to handle a quick rise in salt content.

This means that if you have chosen to do some water changes, do them with the aim being to bring tank water salinity down by several points. 

Oh, one other thing. Above I said that Mg should be tested and if below 1200, should be raised. Without Mg available to the animals, they cannot consume Alk and Ca to build their home. Mg is critical to the action of the Alkalinity ions combining with Ca to form Calcium Carbonate. 

Hardly any of us realized, when we got in to this hobby, that chemistry would be so important. 

Aloha,

Mark