This white progress marker says it can build a warehouse anywhere including Mullheim. But it also states that you still need the required progress marker in order to build a warehouse to those specific locations. So if there is no actual other progress marker that lets you build in Mullheim does that rule still apply? This progress marker really doesn't clarify how you are able to build in Mullheim as you would never have a progess marker to let you build in that area, but implies that it can.
The Warehouse progress marker allows you to build a warehouse ANYWHERE on the board for 2 Thaler, regardless of what the actual cost would have been. This is the only progress marker that allows you to build a warehouse in Mulheim. If, instead, you prefer to build a warehouse elsewhere (for example in Port Ruhrort), you must also have the Port Ruhrort Warehouse marker unlocked to do so. Hope this helps