Slug-like thing...Dangerous?

Found this in my tank just now?? What is it? Has a slug like motion? Help?!
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Fish: 1 Occelaris Clownfish, 1 Firefish Goby, 1 Scarlet Skunk Cleaner Shrimp, 1 Blue Green Chromis.
Corals: Green Star Polyps, Armor of God zoas, Purple People Eaters zoas, Button Polyps, Kenya Tree Coral.
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