The only thing it resembles to me is watercress.
Any ideas? It might not even be edible...
Looks more like a brassica to me, like cabbage for example...grown in non-ideal conditions.
Yes, a mass of Brassica seedlings. As Donna said, they needed transplanting a good while ago. Could be cabbage, broccoli, cauli, sprouts. Not possible to tell from the picture.