Seedling new growth turning pale green

Hello, I have a Northern Lights/Big Bud seedling turning pale green on me. The medium is rockwool (flushed with PH 5 water), the lighting is t-10s, the temp is 70 degrees and humidity is 65%. As you can see by the picture, the color is way off. Another same seedling next to it is fine though.

I have been using water but also added some Technaflora Root 66 yesterday, using the guidelines for seedlings.

What am I doing wrong here?

THX!

your cube is has algae growing on it. this can cause problems for the base structure of ur seedling. Since you have other plants that are doing well im guessing thats not the problem. New growth tends to be a lighter green anyways so I wouldn't be 2 worried about it. It cant be a deficiency since you gave it some sort of root enhancer feeding, but it could be a burn.

How far away is your t10 from the plant? Are you using a heat mat for them? Seedlings like a warmer environment, air and water. 75-80ish

I have a heat mat going, I have 2 t-10 shop light fixtures hanging over the seedling tray about 2 inches away. I could have burned it, it wouldn't be the first time for this noob. We'll see!