Triple-leaf Speedy Chile: what is growing there? Why becoming yellow?

Thank you all for your advice! I used just a bit of epsom salt to spray slightly the leaves after lights went off. Yes, I'm growing in living soil. And yes, it's my first run, so I'm extra paranoid :-D Thanks again!

The cotyledon leaves turn yellow because they are what the seedling uses for nutrients in the first 10 to 14 days of it's life. After that it relies on the soil and you for nutrition. Small shoots that come from the very low stalk you can just snip off with clean sterilized scissors or a razor blade. Yellow leaf tips can mean something depending on where on the plant it is. Light exposed leaves closer to the top usually indicates light stress, while lower leaves with yellow leaf tips could mean a whole host of different issues ranging from insignificant, to the beginning of a bigger problem. For right now i would just say monitor the situation and watch to see if it spreads to other areas on your plant or if it just stays on the lower half of the plant. Shaded lower leaves often yellow and eventually fall off if they can't photosynthesize due to lack of light. See if the yellowing spreads on the leaf in question and keep a photo log of it's progression if any. That way you can post a new question with much greater evidence to present. But overall i think that plant looks pretty healthy and happy.

Then maybe you underwatered ;) It doesnt matter now, first you have magnesium issue and BioGrow or even CalMag wont help. Biobizz calmag is peace of shit ... Second plant definitely will need around liter of water, they are both big enough for such amounts.

Thanks @AsNoriu! If you look at my weekly logs you see I have watered a total of ~775ml for the Speedy Chile in the first 26 days. I usually water when the soil is bone-dry on top 3-4cm. You think I couId have still overwatered with this amount? Also: would Biobizz Bio Grow help for Mg? (that's what I have here rn)

Magnesium deficiency and heavy. Epsom salt to rescue. Most probably wrong watering regime, few times water in while pot was still moist and you have this. Aka over watering ;)