Corals turning almost white

[FishyFishy!]

There are quite a few things that can cause bleaching. Your most common are generally high water temperature, and excessive light/photoperoid.

Stress can also cause it as well. So check all your basics. Look for anything that you have done as of late that might have changed something. I would look at your salinity and ph first. Swings in either of these could cause bleaching.

Bugs and algae's can also cause it, though more rare. Do you have cyano or byropsis or anything?

Start with the basics, and work your way up. Did you do a recent water change? You may have forgotten to roll your bucket and caused a trace element spike or drop. A stuck heater, dead power head etc etc etc. Let us know if you can think of anything.