Water system

Bit of a random 1 my defender is running with no thermo and after about 4 miles reaches 80c then if you turn it off the water pipes are a little hard then if you undo the cap a litte it lets the presure out then remove the cap there is a litte white throuth on top is there some thing rong or am i panicking for no reason ?

Might be totally wrong, but I had that on one of my old cars, was the head gasket.