Most likely This is an emulsion, the oil got into the antifreeze. This usually happens when the thermostat gasket leaks or is changed and a small amount is mixed! Since your head was removed, perhaps because of this! After a while, it should settle on the walls of the expansion tank! Take a ride, see what happens next. It was similar on my Dodge, then I needed a cummins intercooler upgrade
https://boostyourengine.com/5-9-cumm...ooler-upgrade/ because the oil gets there