odd artifact in epub viewer

[apricoto]

I have added a few epub books that have this small rectangular artifact, randomly covering words. This has happened in only a few epubs. So far it doesn't make the book unreadable, I can usually figure out what word(s) it is hiding. But I'd rather it wasn't there.

any idea why it is there? is there a way to remove it?
I have attached a pic of the artifact. It this isn't sufficient, let me know.