old shielded leaf.. if it doesn't spread, ignore it. this coudl jsut be the plant shedding it.
The symptoms are in the wrong spot for Ca / B ( B is a trace element and more likely ph or nute lockout if so) .. these shoudl start near the top, though.
i don't see any perlite in your coco. It may be that the roots are too wet for too long, especially if irrigating before a proper wet-dry cycle is possible. you want 2 parts coco 1 part perlite or similar for optimal drainage and aeration. You can have a successful grow not doing that but you'll have better root growth and more robust roots if you do.