Hey Growmie,
Looks like the seedling is showing the first signs of a magnesium deficiency, did a little research and it seems Stonington is coco coir based, and having calcium/magnesium deficiencies is quite common when growing in coco just add some type of cal/mag supplement when watering and it will fix the problem. Hopefully, this was helpful, and remember ... "Happy Growing" and "Free your mind one puff at a time".