This is a brand-new modern fairy tale causing quite a stir in mainland China recently. Some carefree grass-mud-horses were enjoying themselves on the grassland; suddenly evil river-crabs came out of nowhere to destroy the habitat, ... I heard of it for the first time about half a month ago. It has now made it onto the New York Times.
It is actually political satire, and part of the phrase is vulgar in Putonghua. Google it if you haven’t seen it yet. There is a lot more behind the story, which helps us understand the Internet culture in mainland. Just don’t say the phrase aloud in Putonghua. At least not while you are in mainland China.