I had a clownfish that had white on her bottom lip. It appeared two days after I got her. I thought the same thing, that maybe she had hit it or something. She had been smoothing out a spot in the sand and I thought maybe she hurt it. My LFS told me to watch it and see if it cleared up on its own. Well, it got worse. Her mouth started looking like it was rotting away. She couldn't eat because her mouth was stuck open all the time. My LFS sold me something called Artemiss that ended up being an herbal treatment to boost the immune system and fight off whatever she had. I think I waited too long to treat her, hoping it would heal on it's on. She probably needed to be treated with an antibiotic. She ended up dying 8 days later. I'm still not sure what she had. My best guess was mouth rot. My other clown did get a little white on his mouth too, but his went away. Maybe the Artemiss helped him, I don't know. Good luck, I hope it is trauma and heals up quickly for you.