To OP,
This is definitely chlorosis as stated. Prob as the result of nutrient deficiency. I'll bet if you pot up with some good fresh soil you'll be good to go
If you can provide more info that would be helpful. Soil type, lighting, temp... bugs? It does look like nutrient deficiency of some type.
For MJ,
I'm confused. How did we determine that the soil is too alkaline? Maybe I missed something. Sphagnum is usually acidic (ph around 4) and worm castings are at about a 7. If it were indeed too alkaline.. you also wouldn't want to pH up...
The soil is 60% sphagnum moss and 40% worm castings.
Is all you are using is SM and castings?? If so, that is your problem. That mix maybe fine for rooting cuttings/seeds (and small containers that dry out frequently, and consequently have to be watered frequently), but not for larger plants. You need more aeration/drainage.