Over the years I have experienced birdsnest corals bleaching for no apparent reason. Those of you who have seen my tank know that I too have a couple sarcos (I have a serious mixed reef) that have been there the whole time. The first time it happened was three years ago, I went out of town for a long weekend and came home to find my rather large pink birdsnest totally white. I couldn't figure out what had happened. I got another one at the next frag swap and about six months ago (that coral had been in my system about 1-1/2 - 2 years) the same thing happened...I also had similar experience with a green one I've had for four or five years.
In my experience the corals always bleach from the base which is probably what you are referring to as the "middle". When this has happened, I have found the only way to save the coral is to frag off the live "tips" of the coral ABOVE the bleaching. It seems if there is any of the bleached area on the new frag it will continue to bleach up the new frag.
I don't know what causes this but though it seems to happen for no reason...all parameters in line, no temp spikes, coral wasn't moved or fragged, etc...I'm sure there is a trigger. Why the birdsnest are more prone to it...I don't know. Why I can have a coral for years and suddenly it begins to bleach...I wish I knew! I run carbon to help with the chemical warfare but I'll tell you I highly doubt it is going to stop the bleaching at this point. I just wanted to let you know you are not the only one to experience this. If someone has the definitive answer, I'd love to hear it. Hopefully you will be able to save at least a few frags of the coral.
Good luck!
