What is this?

[Freshwater Sucker Fish]

I'm sure I know the answer and this will soon be a dead thread. But here we go.
Is this Algae?
Hot water and consistent rubbing best ways to remove this?
Can I use a clean painter's putty knife to remove it?
please see attached picture.
Bonus points for the name of what this is called.

Note: water is very hard.

 

[Sgooosh]

looks to me like calcium deposit mixed with diatoms
i used vinegar, it ate through some of it, but with that thick of a deposit, maybe something stronger will do the job

 

[I Like Rare Fish]

Diatoms with some kind of other materiel, probably calcium deposit as @sgoosh said.