Err it’s standard new paragraph indent. That’s why you get it indented when you hit return. Ensure your editor ruler bar is showing (Format/Ruler) and change it to how you want it using the sliders or right mouse on it…
You can change the default formatting to have no first-line indent by going to the Editor tab of Tools > Options…, clicking into the sample text, and dragging the downward-pointing triangle marker on the top of the ruler all the way to the left. (While here, you can make any other adjustments you want to the default formatting using the ruler or format bar. The blue A button on the left of the format bar opens the font options.) Click OK to apply the changes and close the Options.
Your new defaults will apply to all new documents you create (in any project); to change existing documents to match, select them in the binder (Ctrl- or Shift-click to select multiple) and then choose Documents > Convert > Formatting to Default Text Style.