You can view your new text message as a popup on Galaxy S5, so you don't have to open the Messages app each time you receive a new message. To display new messages as pop-ups do as follows:
1. Open Messages application
2. Tap the 3dot icon, followed by the Settings.
3. Go to Notifications
4. To enable viewing new text message as popup, make sure to mark the "Pop-up Display" option
5. To disable popup window for new text message, just unmark the option