When Pages opens a Word document, it attempts to translate that document content into Pages internal document format. Apple does not guarantee that success, because Pages is not a Word clone, and the original Word document remains unchanged, nor editable by Pages.
A Word document uses the industry standard term Highlight for placing a real highlight behind document text. In Pages, Apple has a HighLight Insertion menu item that is not the word processing industry standard highlight that Word uses.
Instead, it is obscured by a different name and inconvenient location on the red-boxed gear menu most recently named More Text Options:

On that panel, is an item named Text Background in the form of a color well, or a color globe (chooser) that allows you to place a preferred color choice behind selected text in the Pages document. That is Page's answer to Word's Highlight, and to remove that kind of Highlighting from selected text, one would use the transparency item from that color well.
If you encounter an entire paragraph that has a Highlighted background treatment, you may be able to use the Paragraph Background inkwell on the Pages Layout panel to apply the transparency treatment to remove that coloration.