When Clementine is with you, she'll shoot it. If you don't take Clementine with you, you have three things you can do in that situation.
1: Shoot the walker
2: Don't shoot and she runs away to not be seen again
3: Shoot Molly and she's not seen again
So to answer your question, yes, you can shoot the walker in the hallway if you DON'T have Clementine with you.
You can shoot the walker, but if you bring Clem, the she'll shoot it for you. I left Clem at the house and wasted that zombie on my first playthrough, after overcoming my urge to shoot Molly.
In my playthrough, I accidentally shot Molly and my head just screamed "What have you done!?" Then I was watching another playthrough where the guy didn't bring Clem, and he just points and clicks. Then the walker was dead. And I was like, ".__. Huh." Curse you lagging mouse!
Another person here who left Clementine at the house and killed the walker with the first shot. I thought it was easy, and I was surprised some people accidentally shot Moly.
Comments
1: Shoot the walker
2: Don't shoot and she runs away to not be seen again
3: Shoot Molly and she's not seen again
So to answer your question, yes, you can shoot the walker in the hallway if you DON'T have Clementine with you.
Yes. I killed the walker with my shot on my first playtrough.