The captain? That's got nothing to do with the quest.
Have you been back to the cliffside caverns, where the red slime blobs are? Did you travel through to the bottom of that area and down into the deeper caves with the green slime dragons? Did you go to the hole in the back of that area, and fight your way past the new red enemies? Did you find Kuno's knife at the end of this area?
If you've done all that, you should leave the cave using the bright light... walk down and out, and teleport back to the campsite. Go back to the building with the quest board and interact with the treasure box on the right side of the counter.
That's the entire quest. Wherever you are on it, that should hopefully help you.