probably a bit hot for the seedline (nutes). Becareful you don't fertilize areas where roots don't exist day after day. it will build up through evaporation even if you have a good 20% runoff if the plant is not eating anything there.
i'm guessing th eroots just hit something a bit too strong for the young plant. could be wrong. if it's clearing up, all is good in the world. blemish early are no big deal, they'll power through it.