There's a number of different objectives within the What a Mess missions, and all of them are covered below.
What a Mess: Clear the Clog
Head on over to the Pump Station in Maintenance for this one, and you'll find a room with two power sockets but no power supplies to insert. To your right is a ledge you can climb up. Hop up there to reach the next room, and put the power supply you find into the power socket in that room to open a second set of double doors with a second power supply inside. Take that power supply and Launch it back into the first room. Then remove the power supply from the third socket and use them both to power the two initial sockets.
Now you need to interact with the pipe up the stairs, which will open the door at the back of the room. Head down to the lower level, shoot the four clog tentacles, and mission complete!
What a Mess: Talk to the Plants
This one is super easy. Head to Central Research simply interact with five plants in the enormous room. They're not difficult to find and the interact prompt shows up from quite a way off, so just visit each floor and interact with each of the plants in turn. Don't forget about the one at the very top, by the entrance to Dr Darling's office!
What a Mess: Clear the Mold
Another super simple one here. Head down to the Medical Wing and look for the bulging Mold growths. Again, they're easy to find, just make sure you thoroughly check every corner. There's also one through the inaccessible doorway at the end of the long room, just aim in and shoot from the outside.
What a Mess: Even More Mold
Now you have to do the exact same thing, but this time in the Coolant Pumps area. Once again, it's not tricky; just check every corner and crevice. Handily, all of the Mold is on the upper level so you don't need to venture to the ground floor. There's quite a few tucked away in the corners of the concrete shafts too, so if there's one you're missing, make sure to check all of those.
What a Mess: Burn the Trash
Finally, for the easiest of all the What a Mess missions, you need to burn some trash. Head to the Furnace Chamber west of the NSC Coolant Pumps, and when Ahti says trash, he really means barrels full of nuclear waste.
All you need to do is grab the barrels one by one, then use Launch to fling them into the furnace. None of them are particularly hard to find, there's one on the lower level hiding in a corner but it's still easy to find.
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)
