What is this yellow dot?

Consistent pH is recommended. I had a look at your diary and first thing i noticed is that your soil looks really wet. That's a no no, especially in their infancy. They only need a little water at first, and no nutrients for the first 2 weeks above ground. Planting a seedling in a large pot right off the bat is a really bad idea, and then soaking the entire body of soil, No! There is a reason people often start their seedlings in plastic party cups. It's a small space that doesn't need a lot of water and is more manageable for you, and the young tender seedling. Seedlings have no root mass. They have a tap root which is primarily used to anchor the plant in the soil. Then roots start to emerge from the tap root and will branch out. Until you have a established root zone, the water you dumped in is just gonna sit there and stifle your plants. They are gonna be water logged and have no ability to get oxygen to their roots. If "properly watering" is saturating your soil till there is runoff, for a plant that size, that's not proper watering, that's over watering. For example, i over watered a Oreoz seedling about a month ago. It got droopy, sad, stunted, started yellowing and developed necrotic spots. and that was in a 6" pot with 100ml of water. To dry out that soil, lower your humidity to like 30%. Raise the temps to 80 to 82F. This will cause the plant to transpire more rapidly. As well it will help to just dry out the soil through evaporation. Get some air moving around those plants as well. The wind will also help to dry out the top layer of soil. Remember, less is more.