Re: 150 hp Force Fuel Carb Question.......
The reed plate assy sits underneath each of the carburetors. Basically it's a row of thin metal flaps that allows the air/fuel mixture sucked into the crankcase through the carburetor to only flow in one direction. There's a couple of folks on this forum that have 5 cylinder 150HP motors. Maybe they can chime in and give their thoughts on the blow back as seen in your video as compared to their motors. I recently did some carburetor adjustments on a 150 HP motor and I don't recall seeing any blow back like what I see on your motor.
Reed plate assy consists of item numbers 27 - 31 in the diagram below. The reed petals (item 28) are what go bad, get bent or broken, and are replaceable.
Mercury, Mercruiser, Force, etc. - FUEL INTAKE SYSTEM - 150 H.P.
Not sure I would concentrate on the milky colored stuff blowing out just yet. If it is water contamination, you will most likely see the milky mixture sitting in and around the reed plate assy....maybe in the crankcase area too when you take off the carburetor and adapter plate. Most likely source would be a head gasket or exhaust cover gasket.