No, mustard greens are not used to make the condiment mustard.
While mustard greens come from the same plant family (Brassicaceae) as mustard seeds, the greens are the edible leaves of the mustard plant, and mustard is made primarily from the seeds. Grinding and mixing mustard seeds with liquids like water, vinegar, or wine creates prepared mustard. The leaves, on the other hand, are enjoyed as a leafy green vegetable, similar to spinach or kale.
In short, mustard greens are eaten as a vegetable, while mustard is made from the seeds of the mustard plant.
