Magnesium, Tim. I have used it for years on my orchids. It seems to help green-up leaves and I started using it on my plumeria last year, hoping it would give those large leaves more color (green). I don't know that it does but my plumeria are awfully lush and green, even in all this heat. I mix the Mg (Epson Salt) powder in with water-soluble fertilizer every spring, mid-summer, and then again in the fall.
You are wise to be somewhat cautious in watering if you don't know about rooting. You should be equally cautious, perhaps more so, fertilizing a plant without roots or with few roots. If I am not seeing active leaf formation and growth, and think I should, I don't hesitate to take that plant/root-ball out of its pot and examine it. That's one way to really know whether good rooting has taken place.
Sometimes we see leaves doing crazy things like you see and don't really ever know exactly why. It could be due to lack of water, perhaps too much water, nutrient deficiency, or stress.