Julie was interviewing Matt after he was evicted. In response to a question about not aligning with Britney, Matt called Britney a "succubus".
I love when stuff like this happens. But I can completely relate to Julie's nervousness over a word airing that shouldn't. If someone said "succubus" on Star, my instant reaction would be to hit the "dump" button on our delay system and wonder about the exact definition afterwards.
The definition of "succubus" is a seductress or enchantress. According to wikipedia, it's a female demon in human form that seduces men in their dreams. There is little R-rated-ness to a succubus, but my guess is Julie was probably just reacting to part of the word - that's what would catch my attention. It sure made for great TV!
The moment of panic starts at the 19:23 point of episode 22 of Big Brother. Click on the link for the full episode and jump forward using the slider to see that bit of fun.