Damping off is a disease that usually only affects seedlings.

If the area around the stem is kept moist, and warm, a fungus (I believe, may be a bacteria) attacks the stem. It looks like it has been pinched, or crimped from being bent over. Once you get it, you may as well pitch the baby, and pop a new seed.

 That's why we say to water around the stem, not directly on it. Your media will wick the moisture over to the roots/ stem. So you use cinnamon to prevent damping off from the moist media.

We've all been there DD !!!! RBT has got you covered. Some weed seeds are more vunerable to DO than others. Air movement helps A LOT, and be careful when watering. Oh yeah, Rbt it's a fungus bruh .