I think it is some kind of fungus/mold.
I would remove the two side branches. Then dab the wound with trichoderma.
If you don’t have trichoderma. I would recommend reducing the stress but cutting the stem off about an inch above the rotting spot. You have enough branches and your air is dry. So cutting a couple of branches off to save the plant should be okay.
Pray that the rot had not spread to the core of main stem. Good luck.