According to this recent post on the PTR Bug Report forums, Unstoppable Force and other conditional sources of resource cost reduction (like Pride’s Fall and Akkhan’s set) don’t interact with the Captain Crimson bonus. It’s currently unknown if this is intended or a bug.
Edit: I found another post with information on Captain Crimson and other new set / legendary mechanics, which you may find useful. It is not affected by Unstoppable Force, but it is snapshotting certain sources of RCR.