It looks like damping off. It can be caused by excessive moisture in the stem area.
I try to keep just the bottom of the growing medium damp when starting seeds. Lack of air circulation can also be a factor.

The pinched stem is damping off. It's a bacterial issue. You don't have to be sterile if you have more good bacteria than bad. Or you can try adding peroxide to your nutrient solution.

I'm not a rockwool fan. I like to use trays of vermiculite to start seedlings. For plants going into hydro, it's easy to pull up a seedling, wash off the vermiculite, and transplant into a net pot.