How long does it take before the coral dies? What are you measuring salinity with, hydrometer or refractometer? How are you acclimating the coral to your tank? IMO if you cannot keep a mushroom alive in your tank and your fish are still alive, I would start narrowing down causes that can only affect corals not your entire tank. Depending on how long your corals live for before they die you may want to start looking at the possibility of a parasite, bug, snail or some other coral eating invert.