It could be one orrrr a few things,...
1. from the appearance, you are growing in soil? Check your soil PH, you want to have it at 6.5-6.8 growing in soil media.
2. You don't want the grow area too moist. I would get a plastic see-through dome, you can use a produce plastic container or cut an old plastic milk jug in half if you need. you want your humidity in the 60%-90% range for a seedling.
3. Don't give seedlings any feed. just mist the area( plastic dome area) with water, rainwater if you have it, distilled if you don't...Start setting tap water out for 24 hours for the gases to escape, or use a air circulation pump to move the air through the water. It all depends on your water, some people have RSO, well water, etc.
Some growers feed in this stage, growth hormone usually. Burr IYKYK