Your pictures are a little hard to see on my phone, but from your description you may have an early powdery mildew. It thrives in cooler temps and low air movement and too much humidity or moisture.
Your temps are okay, could maybe up them 2 degrees, say 23 min and 27 max. A small oscillating fan blowing across the seedlings helps loads. You can carefully wipe off those spots with a most tissue wetted with watter and a very small amount of dish soap. Think drops in a half liter. If they wipe off they are pm, so that is diagnostic.
If you are in an area where you can get sm-90 it's a great all purpose natural pesticide and will kill pm very easy.
You will probably be able to just warm things up and get the air moving across the top of that plant and wait for the pot to get light before you water and be fine. The spores from ppm won't land if the air is moving.