i agree with all the above comments, king is spot on with it been algae, this is caused by the light shining on a wet or damp surface, so its not a problem as it will die as the pot dries out but will appear again but as long as its not over watered then it wont turn into a problem,
now the problem plant should turn out ok, ive had plants like this and by the 3rd set of true leaves their back to normal, it could be any number of reasons why its started growing like that, it could be that it wasnt planted deep enough and the seed shell came off to early as i got the same problem when i remove a seed shell thats not falling off or if i dont plant them deep enough, so it should sort itself out and grow as normal.
everything thing looks good,
nice work