I don't think it's mold.
Weirdly. Are you using anything organic that promotes mycelium growth?
As that stem is dying/ dead, dying things can release beneficial fungi.
Now, I've no seen it like that. But mold I think not. There's no moisture left in that. So, my guess its a by product of degradation.
Just to be safe. Get rid. Use some gloves okay. Even if it's beneficial, it won't make much of a dif.
Also, your running short of mag. Your stems that should be green are showing signs they need mag..
Good Growing