Black algae sounds nasty, I wouldn't risk using it. I store my salt water in a plastic container as well & have never had this issue. I have a small power head running to keep it well mixed & prevent stagnation until I'm ready to use it. Usually don't make up fresh water until a week or less before a water change.
Are you using tap water to make your fresh salt water? If so, there could be sufficient organic material in your mix resulting in stuff 'growing' in there, especially if stored for a long time & stagnant.
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