What would happen if the rotation period of the moon were to decrease

There would be no "dark side" of the moon

Explanation:

That the rotation period of the moon were to decrase means that the rotating speed of the moon over its axis would increase: it would rotate faster.

The current rotation speed is the cause of the "dark side" of the moon.

We only see one side of the moon and call the other side the dark side.

The "dark side" is not darker than other parts of the moon. It is just the face that is not seen from the Earth because the current period is synchronized with the period of the Earth, in a way that we always see the same face of the moon, as if the moon was not rotating.

Threfore, if the period of the moon changed (decrease or increase), we could see different faces of the moon in different nights. No more "dark side".