Sorry I'm late here. That's called corking. Turning to wood. It happens all the time. I try to keep my Euphorbias out of direct sun for this reason. Once it forms, it keeps growing, consuming the whole stem. Turning to hard wood. Surprising it has not effect on White Ghost or red Trigona. It is harmless except for the look of it. Part of nature caused by the Sun.
My large Euphorbia lactea has it all over the bottom half.
