If you want to stop monitoring the serial connection you should press control-a, then control-\ (This tells screen to disconnect, rather than detach, otherwise you'll find an error about the PTY being busy if you try to reconnect).

You'll also need to remember to:

printf("Line of text\r\n");

This might need some additional information:

Backspace:
This doesn't seem to work for screen.

Baudrate
after 'screen /dev/tty.usbYOURDEVICE' add the baudrate number, e.g. 115200, 19200.

Killing screen
This can be a pain for the newer people
Press Ctrl, Shift, a, k

This will ask for a confirmation.