Something setting off my spidey sense...
WOAH!!
How long have you had that piece of Acro??
Alan, doesn't green basal areas of last acro pictures remind you of something ?
LOOKS like the invasive, parasitic, fatal(to coral) Ostreobium algae spp.
AKA Green Boring alga
It's down right sneaky in it's onslaught
"boring=unexciting", it AIN'T!!
It's contagious in aquarium perspective.
I hope I'm guilty of misdiagnosis.
Any case, it's best to fragment into healthy tissue.
THEN biopsy necrosed area.
If in cross-section it's green, Ostreobium is confirmed as pathogen!
Don't want you to incur losses, or incite your wrath ( Valhalla nogal!)
Contact me if you'd like directions/further action options.
Good Luck routing out all invaders.