Pic - what's this on seedling?

These seedlings are 5 days from sprout. Yesterday I gave them light nutes, but I always do that and this is the same strain I have grown before. I have never seen this before, it is on 3 of 4 seedlings. it doesn't look like nute burn, but I don't know what else? This is in hydro and nutes are Humboldt. I have drained 1/3 of the res and refilled with plain water.

Thanks in advance!

I dont give nutes till the 3-4 week of veg, just pH'd water.

What might this be other than nute burn? Because like I said, I have fed this way with the same strain before.