My Meringue seedling doesn't look too happy, ...

Environment wise 40% humidity is a little low at this stage, I would put the dome back on to try and increase it if not already done and get it to around 50-70. A plant will look like that normally for 3 reasons, too intense light, over watered/not enough oxygen to roots or low humidity usually when a plant has had its dome removed or not hardened off before transplant. It’s hard to see if there is any tip burn on the leaf due to my crappy phone but my last port of call if changing any of the above fails would be too lower the EC a touch as I find in dwc you need very low EC compared to all other grow systems. If possible get the humidity back to where it should be, then if it doesn’t rectify itself it’s either a case of raising the light a touch, dropping the EC a point or two and lastly maybe trying to add a little more aeration. Hard to pin point exactly what it could be but hope this helps pal