It's on the base of the taller of the two, sitting just below the rocks (I moved them to take a look). At first glance it looks like rot, but it's firm like the rest of it. It also sits outside in direct sun from roughly 10-1:30 and then under the 40% shade cloth for the rest of the day (our sun is really harsh and my porch gets sun from both the south and west). It gets watered maybe once a week at most, or if it gets rained on I hold out for a bit until it seems like it needs it.
I don't know how long the spot has been there, but it doesn't look like it was there when I got it a few months ago in pictures.
Thanks!
by Tabula_Nada