Patience.
I'm not sure how long the Alk was low but it probably took months of low Alk before the LPS started to show the signs of their suffering. It will take some time for stable parameters to help the coral stop receding and start to recover.
Okay, I'm going to give my advice to each point of the previous post:
Unfortunately, when parameters are good, large/frequent water changes are more often stressful to coral rather than helpful. Stability is good. Change is bad. Lots of new water is a change.
See the logic?
Phosphate(PO4) typically does not increase immediately after removing the PO4 removal media. Give it time. I'm going to go out on a limb here. Now that we can see a pic of the Green Mushroom Leather I believe it's safe to assume that a low PO4 level is probably not the problem and certainly not the entire problem. If PO4 were at zero and had been for a while, that particular coral would not be doing so well. Yes, a valid test would be good. Get a new test kit or have it tested.
In a previous post, I may have suggested (or was it in another thread), the cessation of Benepets Coral food, because it caused problems in LPS for another hobbyist. What the... Well, it can't hurt to stop using it temporarily and if it is indeed the culprit or one factor in a combined issue, improvement might show rather quickly.
The sign of chemical warfare is clear to me. I'm not sure why others can't see it. Look how the Green Mushroom Leather polyps bend with the water flow and they are clearly bending toward the right side of the Chalice. To add to the issue, the Green Mushroom Leather is shading the right side of the Chalice.
Lack of light energy combined with toxic chemicals!
It's okay. You're not alone. We've all been here too. Just a tip: Frustration is easier to handle when the issue is seen as a challenge. A problem that will be solved. That is the
NATURE of this hobby. Mother Nature is complicated. That's one reason I love this hobby (the hobby I'm having to de-emphasize, for purely political reasons
)
Aloha,
Mark
Oh, one last tip. Overcrowding can be a serious issue in this hobby. I believe that applies here. This tank is definitely at the limits of overcrowding. In my experience, all those coral covering the sand may be causing some of the problems. Sorry.
My advice: more open sand - less coral - better growth/health