Hey your ppm is a little high but this issue is your rockwool. I have dealt with this several times. The rockwool is so wet it is suffocating the seedling. You have to make sure you have a good sized tap root coming from the bottom of the rockwool before you put it in a hydroponic system. If possible I would remove it from the system. Shake out as much water as you can and let it grow a couple days until you a have 1/2 to an inch taproot out of the bottom. I just got some rapid rooters which are supposed to have better airflow to stop this from happening. Have not tried them yet though. Hope this gets you headed in the right direction.