I believe your "hair" may be the midrib of the leaf exposed, lacking chlorophyll thus apearing white . Likely will be the last time you see that on the plant. Those leafs are the first to emerge after the cotyledon. Likely exposed to something in the soil just after germination that resulted in this occurrence. IMO
Your plants seem to be exhibiting serious results of imbalanced soil, this should be corrected long prior to screening. Gently washing the roots off and transplanting to a proper soil is my one and only non scrog recommendation. Take it for what its worth buddy! Good luck to ya.
If I didn't know you were a scrogger I wouldn't have bothered. (here anyway)
Diagram showing Midrib:
http://www.bing.com/images/search?q...BE3568A959ED5ADA051A1C652&first=0&FORM=IDFRIR