OMG... that tiny little spot is the start of the dreaded maternalis Overbearius syndrome. It starts when newer growers become so obsessed with any little problem that they can drive the plant crazy.
Obviously this was my bad attempt at humor, but the seedling looks fine. You are going to find all kinds of stuff like that on your journey, including odd shaped leaves and burnt leaf tip. As long as the overall health looks good, then the plant is generally happy. Spots and specks are mostly harmlss unless you have bugs. The hardest thing about growing is patience. You need to let the plant grow without constant changes to it's soil and nutrients. Plants like to simply sit and soak up the sun. They don't really need much food and grow best when undisturbed. I wouldn't water more than a liter, but I wouldn't water for a week or more (depending on your container size... I plant directly into 5 gallon buckets) to encourage the roots to search out the water onhand. When starting, I first fully hydrate my medium before planting into a 5 gallon bucket, then only water around it with a cup of water to keep the soil damp around the seedling. The medium doesn't dry out quickly without anything growing in it, so I don't need to water the seedling for a week or so. This helps to keep fungus and related issues away and allows the roots to spread ot... in a month they should be coming out the bottom drain holes. After that, I water as needed... sometimes a gallon or more a day for big sativas. Use patience and restraint and you should be fine. I would recommmend a lot more light in the future, but you are OK for a seedling. Keep up the good work!