I got one off of ebay like that once. In the pics it looked brand new (SB dual plane M1), when I pulled it out of the box I found what you described on the underside, my guess was very old antifreeze that turned acidic. No experience with the fixes others mentioned, but if you like the manifold I took mine to a good head rebuilder. He bead blasted it, shot in some new aluminum with a tig welder and resurfaced the mating surfaces. Wasn't a cheap fix, but cost less than a new intake.

Good luck!