First thing I think when I see spots is to check for bugs. But it looks like old pH damage, not bugs.
I don't think it's bugs because you also have issues at the edges of your leaves.
The damage is old because the damage area is brownish: dead. Were it light greenish purpleish, brownish, or yellowish, that would be ongoing damage.
I am also saying the damage is old because by the pictures, it looks like the spots are only on the lower leaves.
Starting in coco can be tricky because people want to treat their seedlings with nutrients and watering the same as they do more mature plants. Seelings in coco need nothing more than not horrible tap water. However many times I've grown coco, I get the same thing. If you keep correct pH and CalMag, it will stick to where it is.
Nothing to worry about. All in all, they look like good plants. Coco is great but can be finicky when it comes to pH and CalMag.