I've grown a few fem WW, and none of them ever looked like that. This is just a guess, but I'm thinking CRITTER. Some kind of insect did that. Have you noticed any teensy tiny flies around the area? If so, it could be them. Or mites. Get a loupe or magnifier and see if you can spot any. If so, there's lots of sprays/powder available that won't hurt the girls.
I think we already responded to this a while back. Oh well, you got root rot. The combo of rockwool and your starter peat plug are holding too much water and the stem is rotting. You could just let it dry out and see what happens, but I'm not sure it will make it. You could also try cutting it above the rot area and re-rooting it like a clone. Hope it works out....Don't overwater!
