@ethan8r Do you really want to play Retropie games on a tablet through the USB port? Think how bad the lag would be. In order for you to use the tablet as monitor consider these:Most USB ports on tablets are for charging the battery. They do not take in information from other source other than through WIFI. Those that do, the video information from RPI has to be translated to USB data either through hardware/software hackery wizardry. Then you will need an application on the tablet to properly re-translate the information back to video. These translations would no doubt introduce delays and lags in game playing time.
Please do not post a support request without first reading and following the advice in https://retropie.org.uk/forum/topic/3/read-this-first