$ cd ~/Library/Application\ Support/Postgres/var-10
$ \rm -f postmaster.pid
OR
$ \rm -f ~/Library/Application\ Support/Postgres/var-10/postmaster.pid
$ cd ~/Library/Application\ Support/Postgres/var-10
$ \rm -f postmaster.pid
OR
$ \rm -f ~/Library/Application\ Support/Postgres/var-10/postmaster.pid