Being between the veins it looks like Mg def. But, it looks a little crispy with defined edges, not as faded as Mg def usually is. Ph problems have blotches with crispy edges like that.
What soil and food do you use? They look a little dark green like you might be overfeeding. Sometimes defs like Mg will look like other things when there is overfeeding (salt buildup in the soil). You're kind of early in the grow for a problem like that. It usually arises in transition, or early flower.