If a moon cycle is shorter than a day it appears multiple times, once for each new phase it enters on any given day. I think what makes the most sense is removing multiple instances of the moon's phases and instead it is condensed to a single instance that says something like "2.6 phases."

This is for Eberron, you can see a rendering of the moons here:

This will happen if the moon length is ever less than the length of a day (also fun fact, the moon appears to orbit the opposite direction if it's orbital period is faster than the rotation of the planet!)