That may be a paragraph or character style associated to the text. Create a new text box without a paragraph style, copy the affected text and the paste it unformatted [Ctrl]+[Shift]+[V] into the newly created text box.
This is indicative of a missing font or a missing font style (e.g. Italic, Bold, etc.)
More often than not, it's a missing font style, particularly when you are copying between two documents that have Paragraph Styles that have the same name, but different font specifications. Remember, anything you paste will use whatever Styles are defined in the document you are pasting into, so your font might change accordingly.
If you click your cursor inside the text, and glance at your Character specs in either the flyout panel or the Control panel, you might see square brackets around the font or style name e..g: [Italic].
What you do to fix this depends on what your situation is.