Grow hydroponically?

Hello, I am currently growing some herbs and vegetables in a Kratky hydroculture.

I have three bok choy plants. After they all germinated well and the roots were clearly visible, I planted them in a Kratky system. I used old beer bottles because they're brown. I filled them with nutrient solution, placed the plant in a soft foam pad, and stuck it into the bottle. The roots are protruding into the solution, but I noticed that a kind of white slime had formed on the roots of one bottle. They're also not growing any further. Did I add the nutrient solution too early?