I read both your posts. That mottling of the leaves looks like boron deficiency. There may have been too much nitrogen in the fertilizer you used. However it is easily corrected.

You'll find Borax in the same isle as laundry soap. It is often used as a laundry booster. Borax is 11% boron. Mix 1/2 oz.--2 teaspoons to a gallon of warm water. Put it in a spray bottle and spray the entire plant. Don't use too much Borax because too much will kill the plant but the right amount will correct the plants deficiency

Thank you very much for the suggestion! I will give it a try. Given that the plants are still under the grow lights and not in the greenhouse or outdoor pots, is it possible that slugs infested the potting mix I used?

It could be. The first photo looks like boron deficiency but the second photo looks like it could be slug damage. It wouldn't hurt to sprinkle a little Safers slug bait in the pots just in case

I will give the slug bait a try as well. Warmest regards for your feedback.

Thanks for your feedback. My geraniums fully recovered with a mild borax solution and when I moved them from my under the light tables into my greenhouse. Little beauties now that they're in their outside pots.