When the moon acted like the sun and went behind the horizon. Imee immediately tried to get back into the cave of the First Port and gain all the necessary information from the Giganslik.
Although she already did, it was safer if she could go through it all again and maybe find another clue on how to defeat it.
However, in the darkness, it was hard to see. Imee had already adapted in the dark and was able to see well but in the complete darkness where there is no light, it would be difficult. So before the moon sank, she made sure that she could see the torch inside the cave.
With total darkness, the light created by the torch seemed so bright. So after dealing with a few more slimes, she hurriedly went back to the cave. The slimes are still active in the "night" of the nights so what Imee did was to ensure that every slime was destroyed in the area before concentrating on the Rune.