The Cantonese phrase "M Goi" has many meanings:
- "Excuse me"
- "Thanks" (casual)
Does it have a 普通话 equivalent? And how can I write it in pinyin? Is it even possible?
above comments already address that "Excuse me" / "Hey" are a result of shortened phrases.
So with respect to "Thanks" (casual), this is what i was taught as a kid...
多谢 and 唔该 are spoken in cantonese but both written in chinese as 谢谢
多谢 is used when receiving a gift "thanks"
唔该 is used when someone does you a favour - i understand it to be another case of shortened phrase possibly from 你唔应该... or “you shouldn't have..."