Need some advice for these seedlings - suspect it could be early cal-mag deficiency. Thoughts?

calcium and magnesium are 2 different cations used in the plant.. they are not tied together. you cannot see an Mg deficiency until 30-35+ days after the problem starts, so definitely not that with a young plant, at least not visible. that looks genetic. hopefully it goes away in the newer growth, but when i have something like that continue beyond the initial leaves, it's usually not worth growing. This is why you should plant 133-150% of what you need to fill the space so you can throw out the weak and the defective.