It's to clean up the aftermath of a CO2 deficiency. There's still brown scum on the glass that I can't clean without disturbing the substrate and affecting the current population of ghosts and amanos. There's also brown dust on the MU (or whatever it's called this month), which is inaccessible without removing the entire hedge.
I think I have it all roughly in balance again. Had to replace old bulbs just after thinning out the floater cover and the CO2 wasn't up to the load. I've since replaced the lily pipe for a spray bar angled to maximize O2 - I now have individual surface boils for each jet. The CO2 level has been increased to just below signs of stress in the inhabitants.
The dust in the second picture makes me worry that there isn't enough flow in that area, so I'm considering a nano koralia, though I detest the idea of adding anything else to the tank. It's only a 29g.
Ferts are EI but with frequent testing to keep NO3 at 10-20 ppm. At the time of the outbreak I discovered the test strips had been wildly inaccurate: NO3 was actually 80-160 and PO4 was 10+. I've had a roller coaster ride getting them to acceptable levels but it seems okay now.