At the end of an El Nino the warm pool of water drifts across the Pacific just below the equator. El Nino usually breaks down during the southern hemisphere summer.
There are times when the warmer surface water, ex El-Nino arrives at the back half of the Australian summer. The reef itself slows water movement from the north/south directions but it is fairly open to surface waters arriving from the east. This allows the warmer El Nino to start pooling inside the outer barrier.
If the timing is right, this water is further heated and can trigger conditions where many coral species will "bleach". They eject their symbiotic algae. This does not kill the polyps. Usually the water cools enough so that the algae re-colonises and all continues as it was. Surface water temperature above 30 deg C is not uncommon in this area during summer in any normal year.
Cooling usually happens through tidal flow and storm (including tropical revolving storms) activity. The end of December, start of January, brings spring tides that also effectively mix the water. The tides a month later are also effective. The main cooling effect comes from the south east trade winds that cool that surface waters and bring them over the reef. They also have a strong influence in reversing the East Australia Current. In El Nino conditions the trade winds are greatly reduced exacerbating the conditions.
Making shinier clouds looks like complete and utter hokum. More chance of a benefit arising from having Trump building a wall across the equatorial pacific and having Hawaii pay for it.