Malnourishment or mute burn

Is it tap water? I don't think it's nute burn considering you haven't added any nutes yet, a good rule of thumb is once the cotlydons fall off start feeding or after week 2 of veg. And start with 1/4 strength and add either more nutes or pH'd water based on which way the ppms go. What type of light do you have? And I was having a similar problem a few days ago and found out the chlorine in tap was hurting seedlings along with the fact I was drenching my rockwool as it looks your rockwool looks drenched as well. Rockwool doesn't drain like at all I run a top fed dwc where I had the water pump feeding seedlings 24hrs a day when I switched it to turn on just twice a day for 15 mins ea time I noticed almost immediate improvement. Hope this helps.

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I'd give it some type of nutes... maybe @ least some rhizotonic.

Just water is not gonna feed that plant, its in hydro.

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They don't need nutes for the first two to three weeks. I agree about the chlorine being a possible prob. Older plants can withstand it better but I've had young clones adversely affected by chlorine in the past.
Did you properly presoak the rockwool before use? What ph is your water? By the way, rockwool will always drain to approx 20% moisture as soon as the flood finishes as long as it's not sitting in water. If it is it will always be too wet.
I flood my early veg table for 15 mins per hour but I have my young ones in net pots filled with hydroton which would dry out fairly quickly. With rockwool I'd flood three or four times each light period