If you position the cursor at the beginning of that line, like so
and press ← Backspace once, the hanging lines will all align with the first line, like this:
Note: This suggests that Google Slides uses some kind of pseudo-character to represent this type of hanging paragraph indentation. If you press ← Backspace again, all the lines will shift further to the left of your text box, suggesting that another uniform indentation psuedo-character of some kind. Neither are true characters, in that they cannot be selected or copied, so they are most likely some internal state tracked by the text box.