What is this stuff in my block?

Get a good quality coolant flush and put in 2x what is recommended and then follow directions but run 2x as long as they call for. Flush cooling system with a "T" in the heater hose to get full flow and pressure then fill with clean water then drive 200-500 miles and then flush again with clean water. You will be amazed at what comes out.

I used to run into this all the time on older engines that were not maintained. It is corrosion from all the metals in the engine and cooling system. The newer coolant does a way better job than the old stuff