There are many solutions for accessing GUI applications as another user (in X Windows). Several are described in this excellent post on the Arch Linux Wiki.
However, I needed a different solution as I frequently “su” to different accounts in the terminal to perform different tasks – I normally don’t just start one application and then exit back to the original user.
I ended up finding an answer in the xhost man pages (go figure). The command below should be run as the user who started the X Windows session. Substitute the actual username of the account for the “su” in place of “USERNAME”.
The si prefix stands for “server interpreted”. This syntax allows us to grant access to a single local user. For more information check out the man pages for xhost and Xsecurity.