What it is? Is it calcium buildup or something else?

[bestinla]

We recently moved to house which has a pool.
However our pool has many things that I don't what it is on floor and some on side wall.

I scraped it with a metal scrapper, and I recored it.
I though it was black algae. However now, I think it is more likely calcium buildup.
After multiple scraping, I could remove the build up, however I couldn't remove the residue completely
Please check and let me know what it is.
https://streamable.com/2r43s
https://streamable.com/x6k22

Thank you in advance!