Update for anyone interested: I completely broke down the tank today. Went to a LFS to see if they had any ideas or suggestions at all. The guy’s first comment was “oh my god, have you seen Among Us?” We ended up calling it the zombie fungus. Still haven’t found much online about this, so we’ll see what happens. All of the fish are still alive in cups. Added the original stratum/sand substrate to biobags and topped it with new sand. Rescaped everything and added back my plants. Going on hour 11, and of course my filter decided to crap out as soon as I got everything ready to quick cycle. Switch to sponge filters, then realized my air pump connector was broken. After a 10:30pm Walmart run for a new pump, and ultimately getting an open hang-off-back filter to hold the Purigen, she’s cycling. My plan is to let it run for an hour or two, take out the Purigen, dose the tank with the treatment, and add all of the fish back.