Defiency, light burn/stress, low RH?

With a plant so small it is hard to tell. With your setup, however, the answer may be there: What is your ppm? What is your pH? (also what is your water natural pH?) Does your pH keep spiking? Is your RH good? Did you mist with lights on? At this stage, for say the first 2 week or so I wouldnt add any nutrients but just calmag at say 150 (based on RO or soft tap) and trust in the cotyledons to see you through until they a couple inches tall and the plant is more developed. If your ppm is low (but suitable for the seedling stage) but your water supply base pH is high, you may be inadvertently getting a yoyo effect spiking your pH as when your low level ppm drops this can cause a pH spike similar to adding too high ppm and getting a yoyo from the other side. From the looks of it I would say tox or pH issue and not underfed - under feeding at this stage is hard to do whatever your medium as them cotyledons do have enough food in them to see a plant through until its roots are developed enough to primarily absorb water rather than relying on RH and transpiration through leafs. Not to sound daft, but you havent got your light on 100% at a distance where it should be on say 30%, have you? The leaf taco makes me think light burn or accidental burn from misting but DWC is so touchy it could be setup related, especially at this early stage where variables getting out of whack can have such devastating effects. Sorry I cant be of more help, just so many variables and such a tiny plant it is hard to say for sure, although I would go veeeery easy on nutes for a while just in case you did accidentally dose it too high and aim for like 200-250ppm at the most and keep an eye out for deficiencies or further detrimental effects.