Sorry, but I believe that plant is rotting from the inside out. That is definitely a bad sign. I'd cut the good growth off to re-root while you still can.

Then I'd unpot the whole thing & check the roots ASAP as I believe this plant is crashing. I'd take a guess that it's likely a poor soil &/or bad drainage combination causing the problem.

As Karen says, it could be due to rot.
Could also be the plant shedding foliage and branches that it can't support....if it's not getting sufficient light.
Either way, that branch is a goner.

Josh

I'd cut just above that soft part as pirate and green said. How often do you water it and what type of soil/drainage?