My wife has a small aquarium for a Betta which is a kind of self sustaining little system and grows micro greens in the top. Mustard is one of the most prolific and I agree the greens are delicious. When I lived in the UK over 40 years ago, you used to be able to buy little containers of "mustard and cress" which were good too. As long as the leaves are otherwise healthy, a few nibbles out of them shouldn't be a problem.