It's the stem all the way up from the coco to about 14". When I noticed it I sprayed the plants with BTE to make sure it wasn't some type of bug but it's definitely part of the plant.
Thanks coooper, Whatever it is it doesn't seem to be bothering the plant. I'm wondering if it's the plant trying to block the UV from hitting the stem. I've got the plants pushed kind of sideways with it exposed to the light.