This might be coolant additive residue (evaporation or rather small droplets flung around by the fan at high revs. Looks there is some on the fan shroud too).
Check for leaks (after pressure build-up, when the engine has reached oper.temperature-the upper hose should be firm to touch) on the radiator and the weep hole of the water pump (engine shut, of course).
Also this could have happened if some coolant(or just vapors) escaped via the overflow tube(on the filler neck of the radiator itself).
I'd also change the rad cap if it's the original one (get a Behr/OEM with the same pressure setting).
When was the last time you had checked the coolant level before the purge operation? It might be just a coincidence.