The Morgenthau Plan (named after Secretary of the Treasury, Henry Morgenthau) was created before Germany surrendered, but after the writing was on the wall that Germany was going to. Essentially, the idea was the opposite of the Marshall Plan.

The most important part from a military strategy standpoint was the removal of Germany's ability to wage war. If this meant destroying its economy, that was totally fine. Germany was going to be cut in half, all of its industrial machinery closed, and industrial centers either given to neighboring countries or just to the world at large.

Oh yeah, and the person in charge of the country would be a military governor who could pretty much do whatever.

Really, the Morgenthau Plan didn't end up being that important in the scope of history. It was replaced by JCS 1779, which dialed back the putative parts of the Morgenthau Plan with an aim toward making Germany stable, productive, and not doing the exact same thing that led to Hitler's rise. From there, they went to the Marshall Plan.

As for why all the plans have M names? Total coincidence. Weird though, right?