The potting mix does look a bit woody so N deficiency is not out of the question. The peppers are also very susceptible to overwatering and in the case of relatively big container like this, underwatering as well if you are bottomwatering. The peppers are slow to grow roots so the root system can be somewhat shallow. Be sure to continue to provide bottom heat so the roots grow strong.
If you are seeing fungus gnats, you need to nip them before infestation takes hold. Pepper seedlings with weak root systems can be badly affected by the root-hair eating fungus gnat maggots. Use sticky traps or bowls of soapy water to trap the adults.
For extra nutrients, I would start with something very mild like liquid kelp or vermicompost tea at 1/3 or 1/2 strength. I would also consider UCG (used coffeee grounds) and UTB (used teabags) in the water or diluted leftover coffee or tea. IF you are seeing fungus gnats, you could alternate with cinnamon-infused water or chamomile tea.
The white stuff on the surface could be mycorrhizae in the organic potting mix (which is a good thing — they will break down the woody material into nutrients plants can use), but if this is saltbloom from excessive chemical fertilizer, then that could be affecting the seedling as well and need to be flushed. Note that cinnamon and chamomile which are natural fungicides will probably be detrimental to the mycos — you just have to make a choice.
BTW, my Bhut-types have always been a little bit yellower green than regular bell-types and don’t even try to compare with the dark green pablano-types and jalapeño-types ...and I think serrano types might be more blue-green but I might be wrong about that.
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