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

Head gasket fix-it goo.

Who knows what quack remedy has been used in the rad during it's long history. Just flush it out well, fill with correct coolant solution and try again.
It could be a sign of a cold running engine, doing a lot of stop start city work with a weak or old coolant solution.

You may be lucky, you may not. Bottom line you may find the head HAS gone.

I would say thats a start as in flushing all the coolant and putting in new one BUT. If it keeps over heating and you're coolant isn't empty after a drive its definalty not the gasket and it will be the radiator fan not kicking on.
My car had more over heating problems than ever I know it all trust me…

Sounds like it might be some stop leak compound.