Hi, the green on the pots is Allgue, it's not a big problem it will eat the nutes and oxegen so it's compation for your plant, but mostly it's unsightly.

Like Blunted 1 said, cover your pots. Light + Nutes = Allgue

Do you have a PPM meter?

nitrogen defficency can also be coursed by lockout, if your plants are only 3 weeks old, then they don't need to much nutes, proabbaly half of the reccomended does for this stage of life.