PL/MIX would have been a programming language that runs on the MIX computer. Donald Knuth would have described and implemented PL/MIX in The Art of Computer Programming volume 5 (chapter 10: Parsing), in the future of the alternate universe where he would have written volume 4 and 5 so quickly that they appeared before the MIX computer became obsolete.
PL/MIX would have been higher level than MIXAL, allowing you to use arithmetic expressions of the kind you see in Fortran or algol.
The only evidence in our universe that PL/MIX would have existed is a few mentions in The Art of Computer Programming chapter 1. Since MIX is now obsolete, replaced by MMIX, Knuth will never create that interpreted language, and will instead describe a totally different interpreter in chapter 10. We will probably never find out what XMIX is like.