Trunk issues - Need help

[SenorVerde23]

I'm taking care of my friends babies. ANd today after trimming I notice on the trunk right above the rapid rooters the stem is brownish blackish color and kinda feels woody.
Specs
Veg week 5
Dwc using GH nutes only
Ph today was 6.1. Then Adjusted to 5.7
Temp 74f
There are 2 plants in one bucket. The plant looks healthy over all. There are a little bit of leaves that look like they might have had some nute burn but it's on the older leaves. None of the new leaves look like that. Here are some pics.

 

[iwltfum]

Yikes. That does not look good. I'm thinking it's some kind of late stage root rot. I'm surprised the plant still looks healthy. You could try using h2o2 or some enzymatic way to break down the rotting material, but that looks fairly progressed.

EDIT: Tie a string (or a thread) around the stem where the rotting is happening, and check it in a day or two to see how fast it's progressing. If it's not progressing very fast, you might have a good chance of being able to get the plant back to health.

 

[DukeO]

If it is not mold. It also kind of looks like salt build up.

Hard to tell from the pic.

What does it feel like.

Those rooters could be wicking it up.