Unless it’s there and I don’t see it, it probably would’ve liked to have started in a bit of rockwool, ball of peat, or similar. Helps keep the roots from getting battered around by the clay balls every time you move it.
Agree, are you giving him just water? What pH? 
maybe give some more tenperature if possible to at least getter goin?
Had her on purified water til I saw the cotyledon turning yellow (2 days ago.) Fed a (very) diluted amount not quite sure how much, very little though. ph currently around 6.8-7.
Kept around 73-77°
I’ll try 75-79°
Think it could be wind/light burn?
Most likely over watering… that is a very small seedling to be placed in a hydroponic environment. Running hydroponic’s in net cups with hydrotron clay pallets requires ample feeding in RDWC style or constant submerged like DWC. your seedling needs to have a root system strong enough to sustain the amount of water your chosen media requires. In my opinion, you should let your seedlings get bigger and more stable before you place them into the hydroponic system. Your roots need to outgrow the cube before you should consider placing into the hydroponic system. Take them out of that system let them outgrow the cube… then placed them into the hydroponic system. Cheers !!!
Edit - Always PH 5.8 and low ppm for any hydroponic system’s.
Update*
-Water adjusted to 5.8 ph
-light turned down from 50% to 38ish%
-fan turned off (hoping for 75-79°F)
-fan off pt2 humidity floating from 68-75% hoping for 70-80%
Relying on extractor for the fresh air exchange atm.
Next update in about 12 hours
In hydro pH must be between 5.8 and 6.2 as it’s been said, seedling cannot assimilate nutes if pH is out of range … 