I can't say if that's stuff is the rot, probably is, the fact that they're wrinkled says they aren't getting water, that means the roots or stem is dead...
I'd start whackin away at the most yellow one and see what you get.
It would be a good idea to get that fat round (mamilaria) cactus out of that pot, let him sit around a day or 2 to dry and put him in a clay pot with proper soil, the moon cactus might be able to stay there and be ok, I can barely see it in the pics but it looks ok to me from here...
