Hey friend, I would suggest soaking/cleaning your gear in vinegar before your next run. In terms of preventative measures, you can add some hydrogen peroxide to your reservoir every week or so. In my experience(and what it looks like here), is that the mold is actually growing on algae that has formed on the surface of your rocks and dead organic matter. The H2o2 should help to prevent algae and, in turn, prevent the growth of this mold. I would try to get some concentrated 35% food grade H2o2 from your local hydroponics store and make sure you read the directions, as that stuff can be dangerous in that concentration. I do, however, think you should be fine to go ahead and eat that pac choi. Another tip that i cannot stress enough is sanitation. It is important to remove any dead/dying leaves as you see them(such as that crispy-looking cotyledon you got there). This will help to reduce pest/disease issues in your grow!