The problem is when your puzzle defies the rest of the game up to that point with no real clues. The rest of the game basically suggests you to keep moving and avoid the enemies. Then the mirror wants you to stop. These are two conflicting ideas.
What can you do? Many things. As an example, a book in the shelf of books that mentions a story about the face in the mirror.
Day XX YY
Today, I thought I saw a shadow in the corner of my eye. I turned and, for a moment, I thought I saw the face of a stranger looking back at me. I must have been imagining things since as soon as I faced the mirror, I could only see myself reflected.
You could've also had, for example, a room with a doll facing a mirror and blood leaking from the mirror. This would be subtle, but might be something that the player might remember.
Another way would've been to create a natural way to see it happen. Have it start with a path leading right, then make a turn up with a mirror two spaces up. Have it so that the enemy shows up in that mirror, then when the player turns up, the enemy disappears and your reflection shows up.
There are many ways to do it without breaking immersion.
From what I recall of clocktower, it provided plenty of clues to the player. They were less vague and more subtle. Vague and subtle are two different things.