Yeah, I think Gm's onto it.. Do you have any drainage holes in that cup? It really looks like you've just over-watered the heck out of that thing.. Would imagine those roots are drowning.. let it dry. Is the soil amended with nutrients? If not do like he said and start a small feed. Hope me confirming his suspensions help!👊
Need to let them dry a little on top between waterings. If your not feeding yet they may want a small feed to start. Like 250 ppm 0.5 ec if your in soil and ph should be 6.5 going in. Make sure when watering you have some runoff coming out the bottom of those cups. Cheers