My sanguinea gets that too, not sure what it is either. Also, those are some nice photos of the tubers! I haven't unpotted the rest of my D. ramellosa tubers, so hopefully they haven't dried up. I sometimes give them a little watering, maybe about 1 fluid ounce or so, every 3 weeks. They seem to be a little harder to dry up than you'd expect though, those D. zigzagia tubers you gave me are pretty good being dry in the bag (with the little humidity that there is in there, though I'd expect soil to retain more humidity than what's in the bag).
This reminds me, that I need to find a bigger pot for my D. menziesii, which I also have not dug up. I'll see if I can get you a tuber or two, for September's meet. I don't expect them to start growing then, since they went dormant so late.