I noticed that too, but I started thinking. It is pretty rare for Mother Grubs to renounce brooding, so maybe they aren't considered lusii, but the Virgin Mother Grub is referred to such because she is now fulfilling that societal role now? Maybe the word "lusus" for her is probably used as a title, the same way as "custodian" or "guardian?"

Also the wiki somewhere said lusii have white coats so they can reflect the harsh sunlight off their bodies, so it hurts them less. I think that was something said in Friendsim? Maybe the Mother Grub has a white body for that same reason? Though that now asks the question of why would the Mother Grub go out of the caves during the day, if its already rare for them to do so?

Then again cave creatures irl can be pretty pale, so maybe Mother Grubs are colored white for that reason