As the model revealed in an interview with ELLE.com, that thing is Bieber's...eyelashes.
"I don't consider myself as somebody who has the longest lashes in the world," she explained to the website. "It's fine. I've accepted it. I can live with it. They're visible, they're there, but they're not super long."
However, "My husband actually has way longer lashes than I do, and I get really annoyed," she admitted.
Hey, if that's the worst of it after nearly two years of marriage, then things are going quite well for these lovebirds.
While Justin may have a leg up on his other half in the eyelash department, Hailey has been able to lead the way for him in the skincare department.
The style star explained that the singer has taken a greater interest in skin maintenance under her influence. "Let me just say that my man is so into skincare. I mean, I wouldn't say so into skincare, but he cares a lot about what's going on with his skin now because he knows how much I love it. And he's just taking my advice," she said.
In his struggle with acne, Hailey said she "tried to help him clear that and get that under control and it is working. It's gone, and he's really happy. I give him all of the tips that I use personally and I've given him products that I use. I'm always big on keeping your skin hydrated. It's taken him a while, though."
Fortunately, the Grammy winner has a budding skincare pro living with him if there's ever a bump down the road. "I took a crazy deep dive into skincare," the fashionista said of her quarantine activities. "I'm obsessed with skincare. My mom and my grandma taught me a ton about skincare growing up, so when I had the time in quarantine, I was doing dermatology courses and reading all these books about our skin. I find it interesting and I really care about my skin and what goes on it."