Can you tell me what worm likes to eat leeks? The one I have is black with a reddish brown head.
While it's difficult to say for certain, as many worms could be the culprit, I am leaning towards the black cutworm. You can try treating the soil with Bt, neem oil or even beneficial nematodes.
Here is more information: https://www.gardeningknowhow.com/plant-problems/pests/insects/get-rid-cutworms.htm