What nutrient deficiency is this?

It's really hard to say without full knowledge. But it looks like this is relatively early on in the veg cycle and based on previous grows you posted and the ph, it sounds like you're working with coco as well. While I'm not familiar with coco too much, what this looks like to me is that it's just a little under fed. One of my plants (the CBD one I'm growing) started doing this and it progressed a bit but a feed helped to clear it up. I would suggest at your next scheduled feed to up the food to maybe 3/4 of full dose and see if that helps. Additionally, 5.5ph is the bottom of the pH spectrum thay cannabis can absorb nutrients with in a hydro or coco set up. I would suggest maybe moving the pH of your water to 5.8 or so to give you a little buffer room. If this plant is in early veg like I suspect, this plant could be experiencing mild lockout symptoms as what it needs now is nitrogen which better absorbs at slightly higher ph levels. I hope that helps!! Best of luck with the rest of this Mango