You have first to complete the mission and exit the doungeon.

you have to go inside the dungeon on that island and find the exit on the other side

One option is to walk back out through the instance portals, or to use your hearthstone. If you have used the LFG system to enter a dungeon, there will be a green orb at the bottom left of your minimap. Right-click it and you will see an option to exit the dungeon.

When it says that something is stirring, it means that you've been on the floor to long and to get to the exit quickly