Large dark spot in the SPA

[DingDing0220]

My pool is two years old, not long after the pool built, I notice the pool started has this dark spot at the bottom of the SPA, and no other area in the pool has it. I tried to brush it off, but it won't come off. I'm sure it's not algae, since I shocked the pool regularly, and always keep the the FC between 6-8ppm. Anyone has any idea what it is? and is there anyway I can get rid of it? Thanks in advance.

FC8ppm
Salt 3400
Cyanuric Acid 80ppm
PH 7.8

 

[Texas Splash]

Welcome to TFP! Let's start with:
1 - Do you have any pics of the at area during construction?
2 - Do you have a TF-100 or Taylor K-2006C test kit to post a full set of water test results?
3 - What pool products do you use to chlorinate?
4 - Have you tried rubbing the stained after with a Vitamin C tablet?
5 - Any landscaping work done around that area lately?

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