Honestly at waterbear, I have no idea. I just clean the pool the way my dad told me and I've been doing it like that for years and this never happened. Anyway I found out what I could:
We use:
Clear Swim Unstabilized Granule Chlorine
Clear Swim Stabilized Chlorine Tablets
Pool Blocks - All Season Algaecide
This is the order of how cleaning the pool would go:
Vacuum the pool
Backwash
Rinse
Take leaves etc. out, clear filter basket, put chlorine tablets/pool blocks into filter basket, throw granule chlorine into pool
Start the pump to filter again
So what happened was I followed that procedure above and after leaving the pool for a while and seeing it again the parts where I threw the chlorine and where it fell to the pool floor were purple-black.
The weird thing is that after a day it just disappeared. The pool went back to normal, even when we tried to sweep the areas affected, it wouldn't come off then the day after it just vanished. I really just want to know whether the pool is usable or not and what happened.
It didn't look like the picture provided it was a really dark colour.
Thank you so much for the help!
Note: I don't think anything is wrong with the chlorine, we've been using the same brand for years plus I used some from the very same bucket like twice before this incident and nothing happened. I did read about the metals in the pool, but how would the water just get a lot of metal randomly after nothing changed? That's what I'm confused about.