What is this? Calcium deficiency? 2 weeks old, only on one plant

Google the nutrient deficiencies leaf chart. This will help you to determine the deficiency. It could be a magnesium deficiency. Your plant appears ready to get a feeding. So with the the dialed back base nutrients also add in Cal Mag. It's cheap and should help you a bit. Doesn't look like toxicity but I'm not 100% sure what your lacking right now. Best of luck.

When your plant has a calcium deficiency, the main symptom that you’ll be able to notice is brown or bronze splotches or spots on your leaves. It is also often associated with yellowing (though leaves don’t always turn yellow). Occasionally the splotches may appear purple especially under LEDs. Calcium deficiencies are often caused from low pH at the roots, but also occasionally from growing in soft/filtered water. Typically tap water has enough calcium added to it. Calcium deficiencies tend to appear on newer or growing leaves, which means calcium deficiencies first appear on leaves where there’s rapid vegetative growth, which may display: Dead spots Crinkling Spotting / Mottling Small brown spots Stunted growth Small or distorted new leaves Curled tips Leaf die-off Affected leaves may appear green besides the spots Looks like this is what you’ve got, check your ph/run off, boost calcium and wait, that leaf won’t recover, but watch the new growth for signs of health, or more symptoms, good luck Growmie 😎👍