At low pH in coco your plant cannot absorb magnesium, and what we are seeing here appears to be the start of a mag deficiency.
To avoid this, especially in veg in coco, you need to let the pH go higher. Below 6.0 you won't get enough to be absorbed at the roots so let your pH run from 5.8/9 to allow uptake of manganese, boron & phosphate to a high of 6.2/6.25. Above 6.2 is the sweet spot in coco for Mag uptake.
Keep adding the CalMag at the same rate but once you get the pH right you typically can drop it back to every 2nd feed.