Toilet question regarding fungus

It is likely a result of hard water. Water is described as hard when it has a large amount of dissolved minerals. The water absorbs the minerals from the ground. As the water evaporates it leaves a mineral residue behind. This can be remedied by cleaning with a product that removes mineral stains( CLR, calcium, lime and rust). A more costly treatment is a whole house water softener. It removes the minerals before use.

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!

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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!

Copper corrosion products are one of the few things that make blue/green water deposits. Good news you don't have mold of fungus as copper kills them and most other water life. That is an usual blue , maybe because or the background white. It looks like aggressive corrosion indicating unusual water conditions. A water softener can accelerate copper corrosion. I would ask the municipal water company about it , if you have one . Or ask neighbors if they know anything and get water from the same aquifer. It is not caused by hard water , that is generally protective. It is from your pipes : municipal pipes are iron in any system I know of.