Well, I was right. It does seem to be a new appendage, which is exciting. But, it is growing very very slowly. Here it is as of last month.
As you can see, the bump has grown very slightly, and turned green with hints of purple. But it's far from the little cactus "arm" I expected by now.
I guess I should have known this would be slow-going. In the whole time I've owned it--about 7 years--it has only grown 5 inches. So, I guess I will have to be patient. Hopefully, when the weather warms up, it'll have a little spurt. I'll let you know.