Overfert. It's been written time and time again... FFOF is too hot for seedlings. Use Happy Frog or an inert seedling mix instead. Also looks like the effects of superthrive OD (which is technically an extreme N overdose). Looks very wet too. Flush well, add drainage, cross your fingers, but start more because seedlings rarely recover into healthy plants from a stressor that severe in their earliest life stages.
Small seedlings need a LOT of light or they will stretch and flop. As much light as possible without increasing temps to the burn point.