Ive recently been getting a fishy smell(lol) from my heaters when its moved to the hot side, and the windscreen keeps fogging up with some kinda of greesy shit, also the same day my temp went shooting to the red(water)

then i let it cool down, i checked the water in the radiator and its a dirty green colour but ive topped it up now,
ive been told the heater matrix could be going but if so how does that affect the water temp gauge, ive also been told to check the thermostat but i dunno how these both work or wat they do properlly, anyone know what the problem is?
cheers x