Yellowing veins & New Growth

Hey Everyone!

First time outdoor grower here. I'm growing 5 White Widow autoflower plants from ILGM. All of the plants seem healthy except for one which has yellow veins, yellow new growth, and one yellow spot on a leaf (hard to see the spot in the photo).

• The sick plant is 12 days from germination
• Grown in Fox Farms Coco Loco potting mix with 20% perlite. This is a nutrient rich coco based grow medium amended with earthworm casing, bat guano, etc. Since this is such a hot mix, and my seedlings are only 12 days old, I haven't added any additional nutrients. The plan is to start adding a very gentle nutrient regimen on day 14.
• Growing in 5 gallon pots.
• Watered with .4% unsulphered molasses (1 tablespoon per gallon). PH balanced to approximately 6.

Yesterday, the plant was exhibiting signs of heat stress so I moved it to the shade - the leaves uncurled and today it's back in its usually sunny spot. While the major heat stress symptoms seem to have gone away, the yellowing is still there. What's confusing to me is my 4 other plants have all received the same treatment, were all germinated at the same time, and are the same strain from ILGM. However, all the other 4 seedlings are perfectly healthy.

Does anyone have any ideas what might be wrong with my plant? A nutrient deficiency seems unlikely considering the hot growing medium and seedling age.