I would have to guess....some kind of nutrient deficiency. The only reason I guess this, is because when I use my cloner too long without changing the water, my clones turn yellow.....canabalising themselves. root growth slows. but if I get them in the soil...they green up within a week. Im a noob so don't take my word on it.Someone else could probably tell you exactly what it is, but its hard to tell by just a pic and no additional info.....lighting distance, nutes in soil, water ph ...sorry I can't help more.