The plant actually looks pretty healthy. I use nutes with ff also when they are only 3 weeks old without any problems. I think u are worrying a bit to much. Also-is the light ON “red and blue?” I think your lights are to far away from the plants to process the extra nutes properly. Id put them at 18 inches for a week to see what that does as u continue the same nute cycle. I believe that will green them up and make them more robust. They arent getting enough light imo. Here is a photo of the lights value. You shoukd use the 600 watts model and reduce accordingly. Aside from minor yellowing they look good. So again-I believe to little light is the sole issue.