I was going to say brown jelly disease too.

I did find it to be infectious or at least it found a way to spread in an old tank of mine. Was just a 33G sps tank but wiped out the whole thing. Sounds exactly like what your describing. I never had any caps, so maybe they are "immune". Caps are pretty hard to kill.

Sounds very similar to what I've had happen to my hynorphora, and callendrium. So far, I've been unable to save the hyorphora even with repeated fragging and dipping. Three large colonies are now down to two frags. Whatever it was, it certainly moved large distances between the three colonies. It was the first thing that happened that started a nasty bad stretch there for my tank.

I found that it was difficult at first to remove all the brown jelly. I repeated with longer and longer dip times, but with the same dip strength.
The infection was sometimes noticeable again the next day. It was a euphyllia that was infected in my tank.
It sounds like I was lucky that no other corals were infected.
It was many years ago that I went through that, so I don't remember if there were other things I may have done, but the coral did survive.

Because the skeleton was so massive, it wasn't possible to frag. Maybe that helped.

Mitch

+1 on dipping with one of the available dips.