Chlorosis problem?? Pictures attached

Hi there,

I started my first grow outside this summer. It's pretty low maintenance; I work at a plant nursery so I wanted to see what kind of results I could get going the natural/ organic route. I used a promix and compost soil medium (which was a bit too heavy and wiped out a few of my germinated seeds) but most of my remaining plants look strong and healthy. They are just over 3 weeks old.

The only thing I've noticed is that two of my plants are starting to show signs of chlorosis. It's starting from the center of the new leaves and working it's way out. I looked through pictures to try and diagnose it myself but have so far seen nothing like it. I fertilized once about a week ago using a low organic fish & seaweed fertilizer called Neptune's Harvest (2-4-1). Any thoughts?

*** The attached pic of my only mutant plant.. From the beginning it started growing it's leaves in 3's instead of 2's. Anyone ever seen this before? What ended up happening?