Hi I resurfaced my pool 3 months ago and have noticed the surface in some area going black. Upon close looking it is mainly the crystals that as turned black, not the concrete Matrix. I was told it's black algea . I wasn't convinced it was as my pool chemistry is very good. However I did the treatment no suprise no change. Also tried lo-chlo stain remover no luck. Any one had this experience and any suggestion. I'm concerned it will get worse. Any help is appreciated.