I can confirm that there is a problem. The receptionist tells me that I have captured 9 slimes so far, yet I do have Maelys as a companion.
But the quest is over, because catching more slimes does not help.
Yep, I've just ended investigation and fixes.
After having the slime humanoid executed, as a companion or at the shelter, talking to the guild receptionists get you your rewards and complete the quest. The issue being in having the quest still appearing in the quests due to a wrong variable in a condition.
It will be fixed for next version (v2.0)
In the meanwhile, if you've got your rewards, the quest is completed
Sorry for that issue!