Hi,
At least there is still coolant in the header tank,so ive got a little time for now.
But the header coolant tank cap will have to be removed at some point and replaced,best to buy another first then some how get the old cap off even if you have to break the cap to get it off.Just be careful not to damage the header tank.
The other thing to check from what you say is to take the oil cap off the engine,its on the top of the engine where you put oil in.If you look under the cap and find the same colour sludge then the reason your using coolant is because the engines headgasket is leaking and will need to be replaced.Because coolant is getting into the oil and over time will damage the engine
Hope thats of help.
Good Luck..
Chips.