Pests! You can't say all people hate them, because you read about people who love to watch the butterflies flutter by, but the results of the flutter by's can be this:
This is, or was, a bok choi plant. Its guest was a cabbage worm. So what can one do?
Prevention is always better than cure. Cabbage worms are the larva of certain caterpillars, and these caterpillars seem to be able to smell cabbage plants (and bok choi is a member of the cabbage family) from miles away. The caterpillars cling to the underside of the leaves, start slowly, but can decimate a plant within a day or two once they get going.
If you are not periodically checking your plants, the first indication will be tiny black specks on the stems and leaves of the plant. As soon as you see this, it is best to search the underside of plant's leaves and physically remove the caterpillar, placing it in a cup of soapy water.
Alternately, because this is hydroponics and your plants are not trapped in the ground, you can take the netpot out and spray the underside of the leaves with a blast of water, knocking the caterpillar off. Spraying will also wash away the specs from the plant which will allow you to more easily tell if another caterpillar has decided to home themselves on your plant (because their will be more little brown specs appearing).
As you can tell from the (not very good) second picture, the caterpillars are small and blend in well so you really have to look. These we thought we had knocked off with water, but it took a second, concentrated squirt to send them down the drain. You can also use some soapy water to rinse the leaves, removing the caterpillars that way. Just rinse the plant off thereafter.
Organic sprays are an effective way to make the plant less attractive to the caterpillar and butterfly. A mixture of crushed garlic steeped in hot water, then strained and sprayed on the plant works well against caterpillars. Other options are ginger and hot chilli peppers steeped in hot water or sunflower seed oil with just a touch of dish soap and water. Spraying plants once or twice a week helps to keep these away, and at this time of year in Hong kong, you will find that they become more of a nuisance, so be prepared to protect your crops.