GBTA suddenly bleached, not recovering

[Abominable Reef]

I've had this GBTA for about a year now, and for the most part has looked like this:

About two months ago, I did a large water change, and shortly afterwards, it was bleached out. It still looks like this today:

A Euphyllia I have also bleached out, but I also placed it more or less in the shade, and it has recovered. The GBTA, not so much. It still eats fine, and is bubbling more than it ever did. but I've noticed it has shrunk somewhat too. Any thoughts on how to help it?

Thx

 

[a4twenty]

what type of lighting do you have and how old are they??

what are your water params??

 

[Abominable Reef]

So here it is three months later, and the GBTA is recovering. You can see some tentacles now have zooxanthellae starting to appear:

I still have no idea why it bleached in the first place, but am glad it's recovering. Just thought I'd post an update, for anyone else with a bleach out, that it may recover, but takes time.

Water params are the same:
pH 8.4
dKH 11
Ca 450
NH4, NO2, NO3 all zero

2@ 175W MH 15K iwasaki, 2@ 110W VHO super actinic