cotyledon is yellowing, is she going to die? what ...

light is too far away. clay balls are a poor medium even if just at the top of a dwc or similar setup. i think they remain popular because people think they look cool. They are bulk and if they moved around with a delicate root system intertwined, can easily tear and damage roots - at minimum impede them greatly compared to other options. They 'work' but might as well use something more functional / efficient at its job. 500ppm unless incredibly unbalanced is not the problem here. I've regularly given 600-700ppm from sprout on for 6-7 years and never seen this happen. they survive greater concentrations in the earth outside just fine too. Were the roots reaching the water? is there some sort of drip at the top before it reaches the water? This just looks like it stretched like crazy and inconsistent root zone. did anything drip on it? also looks a little mal-formed. maybe some sort of genetic flaw if it looked funky from the start. unless you paid a ton for that seed, i to would recomend just starting over, but make sure you don't let it stretch so much, and make sure those roots don't dry out while they grow into water reservoir. Yes, it can survive because it is not dead, but it may take longer to recover and get it growing properly compared to a 3-5 sprout time for a new seedling. Of course, this assumes the same cause is not repeated. stretch is screaming for more light -- if too tight it's screaming for less light. find a happy middle ground through trial and error.