There was a postgres update recently, there is a old folder in case something went wrong.
If your forum works fine, you can execute this command
cd /var/discourse
./launcher cleanup app
It should clean the old postgres data folder.
For the memory, DIscourse works this way, it uses the most memory it can, you don’t have to worry about that.
If you want it to use less memory, you can change the db_shared_buffers in the app.yml (command : nano containers/app.yml)