Well, I've got an update.
I went on my vacation to NYC (thanks for asking
@KrisReef haha), and not good news. Whereas I was getting polyp extension to some degree before I left, since I've returned the toadstool has not opened up. Today is the 5th day since I got home, for reference. The shedding had seemingly spread from what I can tell, and as of today I'm seeing algae growing on the thick film it's letting go of, ironically on the side with the most flow.
What's worse, now it seems my big green sinularia colony, which was the first coral I added a good 5-6 months ago and has grown like a week and been absolutely bulletproof, has shrunk up. Same situation, ever since I got home Sunday night. I've blown on it with the Turkey baster a little just to make sure nothing seemed to be on the surface, but no luck.
I did notice that while on vacation my ALK went down into the mid to low 8's- it is usually around the mid to high 9's. Turns out it was a combination of a slight blockage in my dosing pump plus the alkalinity finally getting low. I replaced the alkalinity supplement and now we're back up to the usual 9.5 or so.
Other than that, while I did have high ish phosphates last week, between water changed, NOPOX (mostly for algae that's irritating my Digi), and feeding more conservatively, they returned to normal levels. When I tested yesterday, I got 0 on my Hanna checker, and around 5 ppm nitrate from my test kit.
Other parameters as of my APEX test at noon today while at work:
9.52 alk
460 Ca
1347 Mg
I'm really not sure what to do at this point. I don't THINK the leather is upsetting the tank per se with its potential toxins- I have digis, LPS, zoas, and mushrooms all happy as can be- but something is up, and the Sinularia, which never gets shrunk up for more than an hour or two being this ticked off this long has me concerned.
I'm feeling inclined to iodine dip the toadstool again (no idea what else to at this point besides blow off the algae, though it's new so it doesn't seem like that's the issue) and maybe replace the carbon yet again? I changed it the day before I left, so it's not even a week and a half old, but I'm running out of places to look to find issues.
Any ideas?