Slush,
Looks to be a case of necrosis. Which can be caused from over or under watering. In your case. I would say it's overwatering and causing nute burn. It's really hard to not to over water with rockwool. It has such retaining properties. The nutes are just sucked into the cube and held if the plant is not established enough to uptake the water. Change your res water to plain ph'd water and let it flush itself. Then change to a 1/2 strength solution.

Ga.