That coco is pre buffered with calcium. Your assumption that it could be a nitrogen deficiency is correct. That and a dozen + other elements the plant needs to grow healthy. CalMag isn't going to fix anything. You need a balanced nutrient that has NPK and micronutrients. Start feeding at 1/4 strength and raise the EC as the plant gets bigger and don't overwater as the plant doesn't have much of a root system yet. You don't need to drain to waste everyday with a plant that small.