You can get the Mold Removal mission by completing Old Growth, the first mission for Dr Underhill.
Eliminate the Mold Hosts in Parapsychology
With this mission, you're simply looking for standard Mold enemies roaming around the designated areas. For the first one in Parapsychology, head into the Extrasensory Lab. Right at the end is where you'll find one stumbling about.
Go into Parakinesiology for the next one, and you should find it wandering the top of the stairs. I'm not sure where it spawned for me because all of the enemies lured it away, but it should be in the area pictured above.
Keep going into the Astral Exhibition for the last one. Thankfully the Astral Entity you captured much earlier in the story is still locked away, so you'll find the Mold Host feeding in the corner of the main room.
Eliminate the Mold Hosts in the Ritual Division
Over to the Ritual Division now and for the first one, you need to take the elevator down to the Ritual Office. The first one is right as you enter the final door, so be careful it doesn't have time to attack you.
Into the Synchronicity Lab for the next one, and you'll find it in the top left corner, near the cell with the six abacus.
The last one is in the male restrooms, just to the right of the stairs up to Luck & Probability.
Eliminate the Mold Hosts in Central Research
Finally, head to Central Research for the remaining Mold Hosts. The first is on the ground floor, near the elevator shaft down to Dr Underhill's lab. Along the connecting corridor are three doors marked Lab 1, 2, and 3. At the bottom of the stairs in Lab 1 is where you'll find the first Mold Host.
For the second Mold Host, cross to the other side of the ground floor and enter the female restrooms.
The third and final Mold Host is in a Hidden Location, way up high. Climb the first three sets of stairs and you'll go past a large window in the wall with Mold seeping out of it. Use Levitate to get through, break the window, and kill the Mold Host up there.
Now just fast travel to Active Threshold and head back to Dr Underhill's lab to complete the quest.
Main Missions:
Control walkthrough: Welcome to the Oldest House
Control walkthrough: Unknown Caller
Control walkthrough: Directorial Override
Control walkthrough: Old Boys' Club
Control walkthrough: Threshold
Control walkthrough: My Brother's Keeper
Control walkthrough: The Face of the Enemy
Control walkthrough: Finnish Tango
Control walkthrough: Polaris
Control walkthrough: Take Control
Side Missions:
Control walkthrough: Old Friends
Control walkthrough: Fridge Duty
Control walkthrough: Self-Reflection
Control walkthrough: Mr Tommasi
Control walkthrough: Old Growth
Control walkthrough: A Matter of Time
Control walkthrough: Langston's Runaways
Control walkthrough: The Enemy Within
Control walkthrough: Mold Removal
Control walkthrough: What a Mess (all missions)
