i know this is a old post,just stumbled acrossed it. And dude,the tiny little so called 'white hairs' you are seeing along the stem line are tiny little root shoots ,and if covered up to the codelydons,may progress and make new root shoots. This is common on all cannabis seedling stems. Ur either seeing this,or possibly small trichomes on the seedling leaves,this is common also,i always see them on seedlings. Good luck....