My first thought is this is some kind of larvae. The white substance could be part of a cocoon. Try looking under the leaves for them. If it is indeed a larvae, you can try BT dust that is specifically formulated for that.

Hi katienotes,
I agree with ikea. But, I'll go a bit further. Probably some kind of cabbage worm or maybe a looper but it doesn't look green enough. At any rate, some kind of worm. You can probably get rid of these with just an application of liquid Bt (Bacillus thuringiensis) or powder form. Both are available at your local nursery, garden center, DIY, etc. I bought a bottle of concentrate last year for about $9.00 and apply it with a pump spray. What it is, is a natural control that is non-toxic to mammals, birds, bees, ladybugs, other beneficials or humans. Bt is a bacillus, or bacterium that grows in the gut of the worms and causes the breakdown of the gastrointestinal system of the bug, thereby killing/starving it. It's a great way to control a variety of pests without harming the environment or the good guys and with no adverse affects for humans either. There are different strains of the bacillus for different pests. I'm not familiar with all of them. I think there are some for corn ear worm and other types of worms that affect other crops. Anyway, give that a try. I love raising pak choi because it is so easy to grow and looks so cool out on my market stand. And, it's a great little dude for stirfries. Good luck!