This kinda sounds like brown jelly disease stuff. I had a nice size Acan with about 30 or 40 mouths, start with one mouth gone over night. I then worked its way down in a line to the edge and then started to work its way around. After I tried to stop it with a Iodine dip with no success, I tried to frag it by cutting the bad side off. It then start again as I don't think I cut it all off and after another fragging try I ended up losing the whole thing.
I say Brown jelly as after the flesh died it looked looked like brown jelly or slime. But from what you have described is the same thing that happened to my Acan.