The bag of pool sand is probably new. If it looks green, I wouldn't use it. If it looks clean, it's only 50# so you'll need more. I would not reuse pool sand. I suppose it's possible, and, if it's clean, well, as someone pointed out, it's not new but millions of years old...still, if you can get new sand for a reasonable price (on sale for <$10/bag) it's cheap enough to replace it.
If the old sand is gummed up or full of long green tendrils, abandon it. Period.
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