It might be "damping off", but it's difficult to tell. What are you feeding? Are the roots being kept wet? What do the roots look like?
I don't grow hydro so I'm inexperienced when troubleshooting a plant that small in a hydro type system. If the plant was larger, the problem would be easier to identify for me.
Hopefully someone with more hydro experience who's seen this behaviour will be able to provide guidance.