From what I know of Murphy's (it's a scourge) you are kind of screwed. It will need to be removed with a solvent. Try ammonia first, graduate to paint thinner and acetone. But you should have mopped the floor with white vinegar solution to break up the drywall dust. It works great.
Murphy's is just a greasy film-creating nightmare under normal conditions, plays havoc with future refinishing, and should probably be banned, when there are so many cleaners that actually work well.