Toilet question regarding fungus
Copper salt?
That is a really impressive turquoise.  I have never seen hard water leave stains so vividly colored.  Not only has it left the stain but the color is intense enough to color the water in the toilet.  If this is from hard water then that would mean water from the tap has that blue color.  I would not drink that!
I think there is something in your toilet tank that is giving off that color.  It will either be something that is artificially colored blue (there are such products available, for putting in the toilet tank - maybe a previous owner placed it?)  Or you have something made of copper that is seriously corroding in there.  The blue colored stuff for tanks should not stain so I am betting on the copper.
That color blue would be copper sulfate.  If you are on well water your water might have sulfates in it that are corroding the copper thing (e.g. float rod) in your tank.
Long and short of it: please take a photo of the inside of your tank and post it!

A little more research - apparently the culprit is usually the copper pipe bringing fresh water.
[
https://www.npl.co.uk/special-pages/guides/gpg120_corrosion.aspx?ext=.
https://caitcocares.com/blue-green-tap-water/

Now and then a homeowner will call a plumber with the complaint of
“blue or green water coming from my tap.” This water this color is
rare. What most often causes tap water to come out blue or green is
the corrosion of copper freshwater line plumbing pipes. The discolored
water is often accompanied by a metallic taste.

To get rid of the color you will need to get rid of the pipe with the corrosion.  To clean it off, use an oxalic acid scrub cleaner; in the US some of these are Zud or Barkeeps Friend.  And serious scrubbing!