it is because the seed casing may have been on it too long...Seeds come stock with enough energy for the plant to make it to the surface and put down enough roots to make some true leaves, and then the seedling is done...so if the casing stays on too long, which sometimes happens if you plant the seed too shallow, the plant will still look for the light, but can't actually get to it...you have seen grass that is covered by something for a few days, then grass in the dark starts to go all yellow, because it isn't getting any light, but if you remove whatever was on the grass that is now yellow it will green back up as it gets more sun...and this seedling is the same way...it got a little light starved, but now that the casing is gone the leaves wil green up...so no worries.