It looked exactly like BJD - that has to be what it was. I have chemiclean on standby - I'll keep a close eye on all of my other corals.This looks like Brown Jelly Disease to me. (the translucent brown along the edge of the surviving part of the frag). BJD killed my 1-year-old 6-inch diameter green monti plate completely in 2 days. When it starts, it seems impossible to stop. However, I was lucky, the BJD did not move to a different coral afterwards. No idea where it came from originally.
Incorrect lighting, food, etc should not kill a monti frag in two days. Hope it survives.