From the two recent episodes, I found something interesting, but I might be wrong though. I re-watched St Olga episode from S1 and the moment when the butterfly creatures attacks Henious, her cheek marks appear. Once she calms down and recites the princess rules she goes back to normal.
The monster observer lady from Starfari mentioned that mewmans and monsters can't live together because it will be bad for the monsters. Now notice how Henious was sucking out the energy from the "camera phone meme" MONSTER princess. Most of the princesses at St O's are monsters. Also it seems that some mewman girls can go into the same mewberty thing as Star.
With all that in mind, there seems to be some sort of connection where if you follow order and rules you maintain your humanity. If you follow anarchy, let your emotions take control of you, it seems that you spiral into becoming more like a monster (mewberty form). There are also the gems that are socketed in various monsters (Rasticore, Glossaryck, Baby, Henious' Cerberus) so they are easily controlled. There's also Eclipsa and her own reasons for going after monsters, but that's probably another topic.
Maybe monsters, mewmen, avarians, septarians, waterfolk etc., aren't races in the traditional sense, but rather beings that are formed depending on their state of mind and relationship with magic? Something like how a Lycanthrope transforms from a man to a werewolf when the full moon appears.