I just noticed that I was having this problem as well. I tried looking into the suggestion that Stefan suggested and I think I understand how to make that work for you.
Select the text that is higlighted.
Then go to the Home tab, in the Paragraph section in the bottom right corner there is a paint bucket, when you click on the drop down arrow you see the Theme Colors and other options. When I selected "No Color" It removed the highlight for me.
An oddity did happen though, when I tried to re-highlight the same section then this solution did not take away the highlight and I had to go to the Highlight in the Font section to remove it.
I think this may suggest that highlights from previous versions are treated differently than highlights put into the file from Word 2010. Not sure, but could explain things for you. Hope that helps!