Need help recognizing the problem!

[Kmcc]

Hi guys.
On week 2 one of my plants start to lean down didn't know what's happening tried many things like water uptake turn that down for a bit less checked constantly ph and ppm/ec everything seemed alright the light and temp was good + fan blowing on them.. plant it self looks good just got a problem with the stem here I will attach some images in between week 2 and 3 ..
I'm growing in a rockwool cube in nft for the info..

 

[HugePeckerhead]

Hold her up with something.

 

[Kmcc]

Hold her up with something.

I am now.. but look on the stem it looks weird on the bottom the others dosnt look like that.. you think it will be fine? Isn't there any bad problems with it then?