What causes thick-mashed potato like substance on/in a radiator?

I picked up a free car, it's a 2002 Honda Civic that he said needed a head gasket. (He doesn't know ANYTHING about cars) all he said was that it overheated and just googled and of course everyone says head gaskets.

I brought it home, checked the fluids and nothing is mixed. Oil is just oil, coolant is nasty dirty, but just coolant. I put about 3/4 gallon of water in the radiator to top it off and the car runs and drives just fine. Didn't ever get past halfway on the temp gauge, heat works great, AC works great as well.

The only thing I noticed odd, was this thick white paste looking stuff that seemed like it boiled over or something out of the radiator. I've saw milky oil and coolant before, this is nothing like that. It's literally thick and gritty like mashed potatoes or something. I'm gonna do a flush with the radiator cleaner stuff and replace the radiator cap off the bat, but I'm just curious what that stuff could be?

2002 Honda Civic EX, 159k miles

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