Sometimes white backgrounds can be added when copying and pasting elements, but I haven't heard reports of white backgrounds being added when changing the justification. Can you share your document where this is happening and point out the element?
To remove the white background, you can remove any extraneous CSS or HTML that is not your text by selecting your text box and then clicking Edit > Edit Inner HTML. Then, make sure that only your text is in the text box:
On a a housekeeping note, can you set a category for your posts?