Sup! PH, Alk, and Salinity look good, but PO4 is VERY high for a reef. Ideally, you want it btwn .03 and .1 Corals will tolerate up to .25 in my experience, but you'll see that growth is stunted and coloration fades around/above .25.
Given that Monti Digitatas are VERY hardy, maybe it was digested by another coral or stung to death by a coral placed to close to share the real estate. That's my guess man, since your other LPS and SPS are fine.
And... there are so many factors that could hurt/kill corals. it's a bit of a guessing game some times. Many other detrimental factors could be piling up, besides high PO4, adversely affecting and irritating your corals over time. Large Temp fluctuations? This can cause bleaching in softies such as Ricordeas and Rhodactis and LPS (as red flags). Keeping it btwn 74-77F helps, and maybe rapid changes in salinity (dumping large amounts of RODI right over the corals) can sometimes cause bleaching as well.
My first order of business would be to substantially lower PO4 man. That's your number one problem right now, which will eventually lead to a domino effect of coral death over time. If corals start dying, then the PO4 would
skyrocket, sparking a crash. Right now, you're lucky to be able to pivot without much damage. You caught it early, so you just need to Get the PO4 down to manageable levels, and your reef will be fine.
1. water change
2. PO4 reactor of some kind with GFO media. I highly recommend 'Kolar Labs HC GFO' (they make a 'normal' and 'HC - High Capacity' product). If you do end up using GFO - FOLLOW the instructios and DO NOT OVERDOSE, as is my usual impulse. Scrubbing the system of ALL the PO4 sharply, will also induce shock and hurt your coras just as bad, or worse.
Hope this helps and welcome to the 'Fun' of reefing. LOL
