What Could Destroy Their Immune Systems AND Dissolve Them at the Same Time?

I have been involved with hundreds of industrial environmental projects over my career as a chemical engineer. Typical organic and inorganic wastes, diluted quickly in the ocean cannot do what you are seeing. Also, many areas of that reef showing disease are not close to runoff. I do know something that is proven to have an effect on immune systems and can dissolve calcium carbonates. Many of the pulsed microwave radars atop the hill in Kauai operate in the 2-6 GHz freuequency range. I calculate cumulative, overlapping pulsed power(not averaged over 1 second) of over 1000 w/m2 in the area of that coral reef. Salt water is a great conductor of electricity and has lots of ions in solutions for charge clustering.

Here are some references for microwave radiation

Here is research on electromagnetic radiation effects on water