Postgres.app Could not start on port 5432

You can stop the process by finding the PID with

lsof -i :5432

and then killing it with

kill -9 <PID>

If you are running your server on a macOS machine and installed Postgres through Homebrew, you can stop the current instance like this:

brew services stop postgresql

Then click the Elephant in the native menu-bar at the top of the screen and it should successfully startup.