Hi, I was told by my local pool shop to added some chlorine into my pool. I chose to use calcium hypochlorite but didn't realise it needed to be mixed in a bucket. I know have a white stain on the bottom of my pool. Its been there fore a few months. I've tried scrubbing and brushing it with no luck. can anyone please help me to fix this issue. the pool shop that sold me the calcium has been useless - i've attached a picture for reference. 
