It looks to be a nutrient problem. Are you adding any nutes?
When doing seeds or clones you need very little nutes till they get about 10 inches tall or so. The soil mix could possibly be too nitrogen rich. It seems to be pretty common in a lot of these potting soils. I have very good luck with Sunshine Mix 3 for cloning and seeds and then switch to Sunshine Mix 4 for flowering. The only thing I add during seeding and cloning is B12 anti stress vitamins.