There has been a lot of talk about both "little white starfish", and other smaller starfish.  These starfish are likely Asterina sp., and most are probably harmless algae eaters, while others (usually the larger, more colorful ones) have been noted to be coral predators.  Rather than take a chance, we remove them from the tank when we find them.  A couple good articles on these creatures can be found  here &  here.
 

Mr. Evil Crab terrorized our tanks for months, happily chomping up any new snails added to our cleaner crew.  I finally caught him, and with a sign of relief, added another $150.00 in snails.  A few weeks later, while staring at the tank after dark, I see what - his TWIN!  When we set up the inline frag tank in the basement, we removed a lot of rock from the display tank which was covered in polyps, mushrooms, etc.  One day, while fragging zoanthids, I notice movement in the rock.  Yes!!  It's the evil twin!  We still to this day, have no idea how these crabs got into our tank.  Probably hitchhikers on the initial purchase of uncured Fiji live rock, but not large enough to be discovered until three years later.  There just better not be another one in there!

The Mantis shrimp was a very interesting creature to watch.  We found it when the 75g reef was set up.  Though, I don't know for sure if it ever harmed anything in the tank, it was getting too big and bold for my liking.  The Mantis was removed and shipped to a friend in Oklahoma, who still keeps him in his own 10g tank.

The dreaded Aiptasia anemone!  We had purchased two Peppermint shrimp way back when to control Aiptasia in the tank.  When first put in, the Peppermints happily ate the Aiptasia, and kept it at bay for months.  Recently though, we've seen 3 or 4 of these crop up, and the Peppermints have yet to touch them.  Maybe the shrimp are too well fed?

Neat little creatures.  Often described as "matchsticks" - or "little sticks with a fishing line".  You can find more information on these creatures  here, and  here.