I mean context is everything, isn't it? I don't think moonrunes is necessarily racist. It can be used as a phrase like "it's all Greek to me." But it can be used in racist ways depending on how it's said, yeah. Moonrunes often refers to any language that doesn't use Latin script, so anything from Thai to ancient Egyptian is moonrunes/moonspeak

A much more racist thing that weebs can say might be calling Japanese people "Elevens." It's a reference to the anime Code Geass, where Japan has long since been conquered and colonized by the British Empire. Japan is renamed Area 11, and characters in the show use the derogatory term Eleven to refer to native Japanese people. It's presented as racist in the context of the show, so I always felt weird when anime forums would use it unironically.