In NCIS: New Orleans, 1. am I hearing LaSalle correct when he calls Pride "King", and 2. why does he call him that?
He calls Pride "King" because that is his nickname.
It is explained in the NCIS (mothership) episode that introduced the New Orleans team, "Crescent City", Season 11 Episode 18:
Down in NO, LaSalle explains to Brody how Pride got the name King: When Pride first opened up the New Orleans NCIS office he made a big drug bust and a local crew made him an honorary member and named him King. LaSalle was a sheriff's deputy and King saved his life after Katrina.