Regarding ruach ra’a – can one leave a cut lemon overnight?
Yes, this is fine.
The Gemara (Niddah 17) writes that eggs, onions, and garlic, which are peeled and left overnight (without before being mixed with other ingredients) may not be eaten, because an “evil spirit” rests on them.
This halachah is not mentioned by the Rambam or the Shulchan Aruch, perhaps because they reasoned that the ways of ruach ra’ah (the evil spirt) have changed, yet it is mentioned by the Semak (168), by the Peri Chadah (116), and by the Shulchan Aruch Harav (Shemiras Haguf Vehanefesh), and cited by later authorities.
However, the poskim do not extend the idea beyond those foods mentioned in the Gemara, and therefore with regards to a lemon there’s no problem