Alright, so I can understand why Colon would want to gather more information on ghouls but it seems odd to refuse going explore the hole with the prince and then finally agreeing when the stepbrother parasite joins in. After all, the stepbrother parasite sees Stella as a potential threat to his position, especially if an engagement with the prince goes through, so it seems to me that he'd have some incentive to get rid of both of them.
But I guess I'm wrong about the stepbrother's parasite current intentions. At least for now, it is still building its strength, collecting knowledge and avoiding conflict with Colon. It'll probably still participate in the stepsister's plots, but beyond that he's uninterested in taking any action.
Also, Colon considered it dangerous to accompany the prince, but then was perfectly fine with them all splitting off to explore.
It seems the monster we had seen before isn't the mimic ghoul at all, and it got defeated easily. Now Colon has also revealed the true nature of the mimic ghoul or at least the monster that is mimicking humans that was in this cave. I'm not sure the other ghouls, that replaced and killed humans that the prince referred to, are the same species of monster.
It's somewhat strange that it's made of sand and that in both cases, no harm came to the prince. It might not like rain, or water, much. I wonder if it even eats anything; I mean, I assume the pile of bones is from the other monster feeding.