Hello! Can someone tell me how to remove this blue highlight from my text box? Thanks!

To clarify pix's answer, you get this highlighting (if enabled, as he notes) when you have a font variation that is a spot or local override. Since you should NEVER use local and spot formatting in InDesign, I'd recommend leaving the feature enabled and fixing the actual issue with your text. 🙂 (Create a proper paragraph style, that is, instead of overriding another.)

This is a useful indicator of deviations from assigned styles.

Click on this icon to turn the indicator on or off.

(The better way, however, would be to control the deviations via styles).