Brown jelly....it's a bacterial infection i believe, and is a notorious and exceptionally fast killer of acans and other mussids such as micros. It's very easy to control though, but it does tend to occur in tanks with off-balance levels.....if you've ever seen it in action you'll know it can wipe out an entire acan colony, literally overnight. I had one buddy lose a good percentage of his aussie acans this way....it was only after fragging and treatment that he able to curb the spread.
that being said....in the above case, i do believe it's the elegance coral that has stung the acan.....elegance corals are capable of inflating to a very large degree, so i do think it would have been pretty easy for it to reach acan...cataphyllia corals also have one of most potent stings around; an acan lord is no match for an elegance.
Zach