No, there is no way to see the code of movmedian. Most likely, it was never implemented as an m file anway, just as C code (or Fortran or whatever language Mathworks used to develop it).
It is mathworks choice as to what functions you can see the code of. Some are implemented as plain m files that anybody (with a matlab license) can read, some are implemented as p files, just so you can't read the code, others are built-in, for speed reason.
If you want to see some moving mean code, there is most likely an implementation available on the file exchange.
