Sometimes if the branches are thick they can block the light from getting to the bottom and cause some tissue death there.
IME the part that is blocked from light becomes more a faded color-that pic looke like no tissue
I also noticed the side dying is the side that not getting direct flow from the power head. I don't know if that has anything to do with it dying that just something I noticed.
The direct flow has nothing to do with the dying. You mentioned;
the only thing I have been doing different is adding without measuring Brightwell's Aquatics Alkalin 8.3 KH buffer cause my ph drops a lot
If this is anew process it is what I would say is the cause-Alk was my 1st guess since its nearly always the cause-it could be this piece was in the way of the direct dosing, or maybe something else but I am putting my eggs in the Alk basket.
More than likely it wont "encrust" back over, it for sure wont rejuvinate.
I have had to (more times than not) frag just above that dead part otherwise it just conitued to die off. Not saying that is what you will need to do, but I have watched more corals die off while I waited and watched to see if it would stop.
Frustrating, but better to save whats alive than to slowly watch it progress-give it some time and watch it closely, if it is continues to keep dying, frag above the dead area-but most importantly, watch the levels of the KH and get them stable as quick as possible
Here is a pic of one of my torts that used to be a colony, it got ****** when I moved it into my new tank since the Alk took some time to dial in with my CaRx- I had to frag it up and have been watching this spot (bottom right hand side) to see if it is going to stop or continue to receed.
Not fun but rather save some than lose all-
