Nualgi is not only safe for corals but helps them open up and grow. Nualgi produces diatoms which in turn consume CO2 in water. CO2 and water naturally react to form Carbonic Acid which is known to increase acidity in water and is the chief cause of ocean acidification and depletion of coral reefs (http://ocean.nationalgeographic.com/ocean/critical-issues-ocean-acidification/).
By consuming CO2, Nualgi improves the pH level of water which helps the corals thrive. Nualgi also produces diatoms which consume excess nutrients in water. Nutrients can promote rapid growth of algae and other organisms that can smother corals (Jones & Endean, 1976)