I got concerned about the corrosion issue some years back on the racecar...
$$$ Alum heads, intake, radiator, ft cover.
Always ran distilled water, Water Wetter, and a tiny bit of antifreeze.
Dangled the sacrificial anode in the radiator during the winter months.
Almost nothing on the anode.

I must accidentally be doing something right, I guess.

If I were the OP, I'd have that block cleaned like crazy, but I'm a bit mystified at the goop he found.