750 ppm is way too high for a 2 week old seedling. On the one hand the spots could point to a deficiency, but the plants is a little young for that level of nutes. I use plain water for 2.5-3 weeks, then go to 300 ppm. At 4 weeks I go to 400 ppm, etc, until at 8 weeks they're at 800 ppm. So you're almost at the same level of nutes I'd give a full-grown plant.
What is the ph of the water?