For starters, if tank is new or newer, it already suggests an issue as they are best placed in a mature tank of 4 -6 months as less than that, tank is subject to chemistry changes, nitrate spikes and elevated phos and/or nitrate.
Another reason will be too much water flow and too little light.
Too much flow and they hide and too little light, they dont have enough production of inner algae known as zooxanthellae which is their color and energy source.
Aso assure salinity is not too low and temperature has not become warm