One plant showing white dots. Possible reason?

That's a happy plant... minor blemish.. observe for and spread.. if not ignore. 8" is a bit close, but you'd see other issues if it were light burn. it would not take long to progress. Looks too big, but be safe and look for bugs under leaves... highly doubt it given shape of discoloration. from diary - "Watered the plants 0.25L with nutrients." you should really water until you see a little runoff, then wait for top 1" to dry before repeatingn process. if you are trying to microdose watering, this may be a cause of such spots due to what happens in soil. a little runoff helps maintain consistent pH and nutrient concentration levels in substrate. Avoid dry pockets which have volatile pH swings and nute contents etc... helps avoid root rot.. helps avoids pests. Also, helps roots grow deep instead of superficially. Don't try to spoon feed everyday in this particular substrate.

Could be nothing, But with the serrated edges of your leaves curling up it could also be the start of light burn. Your manufacture says to keep the light at 16" and dimmed to 50%. From what I can see you have it at 15.5 and 50%. I would keep an eye on it and if it gets worse then raise the light a bit. Happy Growing

Nothing to be worried about, probably just PH imbalance.

I think its nothing to be concerned with just maybe where a dash of feed has been split and turn into the salts that it is made from but to be sure look on the underside of leaves and russle the top of the soil and look for any activity I doubt it will be anything nasty but with soil you just never know :)👊