Many ailments look similar, especially in the beginning stages, so all you're going to get is guesses unless you provide additional info. Information that would help would be:
What medium are you growing in, soil or coco? If soil, what is the brand and mix? If coco, is it straight coco (e.g., coco brick) or a coco mix (e.g., Cocoloco)?
What do you feed the plant and what is the pH and EC?
How often do you water the plant? A non-interpretive answer is "when the pot is 75% lighter, usually 3-4 days later;" an ambiguous answer is "when the soil is dry," as that means different things to different people.
What is the temp and humidity in the grow space area at canopy level?
What stage/age is the plant?
What is the wattage of the light, height, and dim setting? Even better is provide the PPFD if you know or have a way of measuring it.
A pic of the entire plant helps. Also, in the second pic, the leaf underneath the one you're concerned with looks to be deficient in Mg, but again, more info and pics would help.