“Find the Second Memory” is an objective in the mission Paradox Daily Heroic in Destiny: The Taken King
With the Taken defeated, head over to the left of the door that you used to trigger this fight. You need to use the secret passage to the Gorgons' Labyrinth (For those familiar with the raid Vault of Glass).
Head down this passage, duck and go over to the left. Drop down to the area below. Make your way down to the second drop and then onto the third.
A while the third drop you will find this lighted pillar. You want to jump up onto it and head into the passage behind it. Head around the next corner to find the second ghost. It is against the left-hand wall. This is just after you get
Now you need to get out of here.