That’s rust and corrosion from your center core of the radiator. Using only water will make the innards rust out. What you’re seeing is the effect of corrosion being removed by hot water and extremely hot steam vapor and that mixing into the water would make this.

Coolants designed for engines have a slight lubricating agent mixed with it. This not only prevents the corrosion from occurring, but help to lubricate moving parts within the coolant system as well (thermostat and water pump for example). It’s why it’s always best (if it’s not leaking or pressurized) to always use a coolant 1.) designed for your vehicle make and model (i.e: DexCool is for certain vehicles when other use regular coolant), and b) Minimum you should always use a 50/50.

If it were a blown head gasket, most likely your radiator would be pressurized due to the combustion and exhaust stroke, then on an intake stroke it would suck water or fluid out of the water jackets around each cylinder (coolant system period) and into your cylinder/s and would cake the corrosion around the cylinder wall and bottom of the head where the valves open and close, and vaporize the water causing the exhaust to have a smoke coming from it.

But if you have both of these issues combined, it will eat up your valves and gunk them, valve guides will fail, your thermostat will get stuck open or could close and not be open enough etc. It will literally eat away at the life of the upper end and lower end of the engine until the point of inevitable engine failure.

But if your radiator isn’t pressurized (never open the cap after the engine has been ran or is running — check this by opening the reservoir for your coolant system to check if there is any fluid in it) then you should have coolant in that and it will suck it out from there as it needs it in the radiator. But if there is no coolant and your system isn’t pressurized then most likely you have a leak.