Flourish Glue is just cyanoacrylate, which is the exact same as most super glues. In my freshwater tank, I just bought cyanoacrylate from the Dollar Store ("Super Glue Gel") as it is the exact same thing and it worked out well, it cures immediately in water and there was/is no harm to fish, snails, shrimp, or plants. I have not personally used it in salt water but I have read that it is safe to do so. It does turn a slightly cloudy color when it cures, like the color of a little milk mixed with water, so it is not completely clear.
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