Christy,
Someone on another board suggested Predaea weldii, too. After Googling it, I found the exact pic and quote you just posted
Don't think that's what my blob is, though. It doesn't have fluorescent tips either in full MH plus actinic, or in actinic only lighting. The frag retained its shape when removed from the tank for about a minute while I got a bowl and some tankwater to put it in. At the time of this writing, the frag had been in the bowl of tankwater on a cold window sill for the last 3 hours and it's still rataining its shape. 15 minutes ago, I drained the water. Just checked it and it's still got its shape.
This thing has not stung me through all the handling both in and out of the tank. It hasn't moved from the rock since we first started noticing it about 4-6 weeks ago, when I thought it was a small mushroom. It does not retract when touched, nor does it retract at night. Doesn't get bigger at night, either.
In the first video, you can better see the bubbles inside the blob's polyp-y things. It seems to be more animal than vegetable, but now having accidentally fragged it, I'm not sure it's even animal anymore.
I'm stumped!