if it was a bright green it was most likely copper. Just because a pool store said there was no copper in the water doesn't mean there wasn't. We all know that their testing can be suspect! the fact that a seqestering agent took care of the problem (sort of--the metal is still in your pool and can return) indicates that the problem was probaby metal induced. What type of treatment did you use for the Algae? Many algecides are copper based!
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