What IS THIS STUFF?

Have posted several times with questions about the timing chain cover change out on my 1965 273 engine. Now I need some more HELP!!
The intake manifold is an aluminum Mopar. I pulled the thermostat housing off and saw what looked like a build up of scale that was coated inside of the manifold. The stuff is off-white soft and I can scrape it off with my finger nail when wet. When I pulled the radiator hose off the thermostat housing the neck of the housing had this build up on it also. Had to use a wire brush and scraper to get it off because it had dried. From what I can see there is none in the block.
I have thought about removing the intake and trying to clean it out by soaking it in a bucket of vinegar or radiator cleaner.
Does anyone know what this is, ideas on cleaning it out and how to prevent this from happening again?

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