Pulsacoil 3 water tank: water is green

Which cistern tank ? Toilet or feeder ?

What you have seen there is the header tank The smaller tank is a header tank which is there to fill the heating system and provide extra space for the water to expand when heated. This is normal to look like that. Best to clean it because that stuff can get inside your heating system and couse problems. The bigger tank is your cold water storage tank that supplies your immersion where you get your hot water from and cold water in your bathroom. If the water there is green then I don't know what to say because in 14 years of plumbing I never seen it or come across.

Yes it is the Header tank, feeding the Heating. I see the answer is same as mine so I will leave that there.

The Gledhill pulsacoil is a thermal store hot water cylinder. The water in the header tank is used for the thermal store water in the cylinder. The water in the header tank is not used by any taps or anything else in the house. As the water in the header tank and cylinder that is used as a thermal store. Is never fully replaced it will go green after time. Like any standing water. I think it's possible to change the water used for thermal store but you will need to follow the manufacturer instructions Service and user manual can be down loaded on line. Google Gledhill Pulsacoil cylinders instructions manual. Hope this helps