well i dont grow hydro so you need someone that does grow hydro to help you more then me, but by looking at your stem it looks like its rotting, are you feeding yet? whats your ph whats your ppm? of the feed? but to me it looks like your watering to much and it causing the roots to rot, but i am pretty sure if your feeding to strong you can burn your roots as well im not 100% sure but i think you should be feeding like 300-400 ppm not 100% sure, can someone with more knowledge on hydroponics jump in and help him?