Seems like you're giving them too much water. However, that's quite odd for coco, since it's airy like.. popcorn?
Well, it could be a starting nitrogen def. but that would show in the leafstems as well. These should turn purpleish in that case.
What's the PH of your water? Too acidic (low PH) could wash away calcium and magnesium. But these elements are often non-existent in coco around here. Some coco are made just from plain cocos, and although the packaging states 'with added nutes' these soils are virtually empty.
Personally I would start off by using allround fertilizer (if possible on a bio pellet basis, because these don't wash away as easy as liquid nutes) just to spice things up and get them to grow a bit better. Check your runoff PH or use a fertometer (a device that measures the EC) to check if you're in acceptable ranges. Adding cal/mag might be a good idea, but before you start using that, you'd better check the actual status of your soil.
The colored leaves won't go back to normal, but that should help prevent this case from spreading throughout the entire plant.
This is one of the reasons I dislike coco grows, I just can't tell what's wrong without using every bit of knowledge I can recall at that moment.
Also, keep in mind that the lower leaves get waaaaay less light as the upper ones. So these leaves tend to die off for no other reason than light deficiency. It's just the plant saving energy. Which is a great thing! So I wouldn't worry at all if the problem doesn't spread.