Yeah, this is definitely iron deficiency, or sulfur.
If you're using just peat moss and perlite and worm castings, there really isn't much "dirt" which has all the elements like iron in it. Normally iron in soil comes from the clay and silt, which makes up most of dirt.
Iron and sulfur deficiencies are hard to tell apart because both iron and sulfur are used in the same protein,
ferredoxins, which is used in the photosystems to channel electrons in the light process.
Try to get some iron sulfate and add about 0.5g to some water and pour it over the medium.