Slime Hutch
One of the abilities of the slimes is the merge ability or the ability to combine with one another to create a bigger slime. They might be big right now but they don't continue to remain as one and will detach back to their smaller bodies once again after some time. According to the almanac's bestiary page which is part of the Goddess of Taming's book page, slimes do not want to last long enough to merge together due to their individual cores merging together if they are merged as one, and the slimes, they don't want to lose their individuality and become a single entity. They only use this tactic to scare other enemies like their local predators and adventurers who are just starting and would be afraid to deal with bigger monsters.
And now, that seems to be the case with the slime that we are seeing right now.
"What is going on? Princess? Why did they suddenly merge?!" Julia is now readying herself to battle as the slime now appears menacing.