It can be tough to say without tank params. Could be low alk (this can also cause sps to bleach), Mag could be off etc.. You also dont mention how deep your tank is. Or how long your lights stay on. So from the info it's tough to say what your problem could be. What sort of husbandry habits you have etc.. What type of corals you have in your tank, how much water volume you have, flow. There are many many things that could cause it to bleach. Even poor acclimation to your tank. Temp swings, PH swings, It could be you have too low nutrients for the acan as well. You could move it to the shade if you feel it's getting too much color. Or you could spot feed as well, this can help them grow and color up. Acans do like to eat a LOT. But there is an entire list of things you need to go through in order to come up with a serious answer. People here are willing to help, but to get a good answer you should try posting all of your tank info including test numbers. Also pictures of the tank and the coral sometimes help people to diagnose problems as well.