Somehow killing GSP on back wall

[Der]

Ok so apparently I'm an outlier here because I am somehow killing all my GSP, but everything else is doing alright. Things have ebbed and flowed in my 13.5 gallon. The tank is now 14 months old and I've learned a lot along the way. About 7 months in, I introduced some chaeto and bottomed out my nutrients. Developed dinos and things suffered. Through manual removal and more diligent testing, I've gotten away from the dinos and removed the chaeto from my tank. Parameters have gotten more stable, nutrients staying fairly low. So back to the GSP... It was doing great during all of this and the 3 small frags I bought at a swap show had grown significantly. I introduced these to the tank around 5 months in and glued them to the back wall of my tank. For several months, they were growing great and extended several inches along my glass in all directions. Now... For the past 1.5 months, the gsp has been mostly closed and slowly degrading. It's been weeks since being fully open and finally a brown coating starts to take over. I cannot blow this brown stuff off with a turkey baster. It takes brushing it with a toothbrush to get it off and I feel like I'm scraping off some flesh from the GSP too. I've now officially lost 2 of the 3 original frags and the last is showing the exact same symptoms. In some of the more recent photos, you can see how these frags have grown and lost so much. Another thing to note...at night I can see several amphipods walking over and possibly? eating the GSP. Not sure if that's possible.

Any thoughts/input would help! Here are my more recent testing results. Sorry I don't have all parameters...

I also tried to put in some chronological pictures that I've grabbed over time. The focus wasn't always the GSP but you should still be able to see the transition.

Thanks!

PH = 8
dkH = 7
Calcium = 352
Mag = 1247 (tested ~1 month ago)
Nitrate = 5
Phosphate = .064 ppm
Salinity = 1.025

 

[Tahoe61]

All your sps and lps look great, obviously the issue is with the gsp only. What type of lighting? The nitrates are low, is that a newer issue? Can you give the gsp more flow?

 

[Der]

All your sps and lps look great, obviously the issue is with the gsp only. What type of lighting? The nitrates are low, is that a newer issue? Can you give the gsp more flow?

Yea it really is only affecting the GSP for some reason. Nitrates are a little low right now. After removing the chaeto, I dosed nirates for a month or so until I was reading ~20. I've cut back on my dosing and this is where my nitrates seem to have settled. I've actually been micro dosing neo nitrate the last couple of days to see if a small increase would help it...no luck. The GSP honestly gets good flow. I've aimed the return pump to flow right across it.

 

[JerseyReefer]

So I have a large GSP back wall. It’s over 2 years old. It has not opened in almost 2 years. It’s still growing but is basically a black/dark purple mat on the glass. I’ve done ICP tests and most of my tank during the past 2 years has been fine. It’s odd that such a wide spread easy to care for coral can’t succeed in a mixed tank full of harder to care for corals. I’ll post pics later.

My thinking is when I first started this hobby nearly 10 years ago, easy to care for corals thrived in my tank. Now I struggle with the easy ones. Perhaps this is due to lower nitrates. I now monitor and control all parameters consistently. Back then I bet my nitrates and phosphates were out of control. But who knows since I didn’t do much testing back then.