It's the old problem with multiple wives - some are bound to grow jealous of another if they feel that their 'status' is threatened'. Even if Gyokuro was the 1st wife, and Akasha the 2nd, we all know who the
more important wife was.
Coupled with the fact that Akasha's a Shinso and Gyokuro most likely is not... And then another point is their daughters. Even though Gyokuro has two children by Issa, neither of them have silver hair and red eyes like Moka does.