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Wow idk is it salt water I'm not familiar with salt water. Had something like that in fresh water once but it was tiny like a pea size. That looks huge.
It's a flatworm, but you have no idea if it needs to be removed or not, because there are many types of flatworms, some good, and some not so good. I have seen this large one before in some tanks including a few of my own systems. I have never seen it do anything detrimental.