I saw something floating around and when I looked, it was a Duncan head. it looked healthy too! I should've grabbed it and try to let it grow somewhere...
That looks similar to what happens to Frogspawn heads, where they separate from the skeleton and just drop off. I've had that happen to some heads on my Duncan and the rest of the colony was fine.
I had this exact thing happen to my duncan colony a few years ago. A few other lps also got completely wiped out. I had brown jelly disease on the other lps, i didnt notice it on the duncans but im assuming thats what it was. I ended up cutting back the 30 colony duncan coral to 3 heads and it survived. Its now got 15-20 heads. Good luck
No I did not figure out why it happened. What was left I moved to a diff tank. Then I upgraded and they all seem to be super happy now, even the little head in the crack. Oh it was the frogs and hammers along with the torches.