Ok, looks like I discovered what it is. A mutation called VARIEGATION.
From
https://www.mrgrowit.com/post/variegation-in-cannabis-plants
Variegation in cannabis plants (or plants in general) simply means that certain parts of the plant such as leaves and stems vary in color from what is considered normal. For instance, if a plant has green leaves all over except for one part of the plant that grows white leaves, then it can be considered variegation. Genetics can be a cause of variegation, but it is certainly not the only cause. In cannabis plants, variegation manifests itself as two-toned leaves and also albinism. Variegation cannot be treated or prevented since genetic mutation can be the cause.
Variegation can also have pathological causes. But in this case, variegation does not show any peculiar patterns. One of the viruses that cause pale color markings on cannabis leaves is the tobacco mosaic virus (TMV). TMV causes leaves to develop uneven yellow stripes, but unlike variegation caused by genetics, this one is not completely harmless. TMV also causes leaves to twist and curl. A lot of research is required when it comes to TMV in cannabis plants and many growers simply deny its existence in cannabis.