It’s hard to tell since mustard belongs in the Brassica genus that includes other cousins cruciferous vegetables, mustard plants or cabbages. Some of these look similar at the early stage of seedling. I’m wondering if yours could be either Rapini/Broccoli Raab (Brassica rapa), or they are broccoli and start to bolt (flowering early)?
If these are vegetables, before consuming please have the local Extension Agricultural for a confirmed ID. For now check out these included for some of the cousins of the mustard. Best wishes!
For future reference please include ‘where’ (State, city, etc…)to help narrow down the plants ID for you.
Link to Rapini.