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It's most likely some type of nudibranch. It is very hard to say w/out a pic exactly what it is and if you should remove it. You may be able to get an ID here- http://www.wetwebmedia.com/nudibran.htm
To me it sounds like Stomatella varia. It actually has half a shell. It's a very good algae grazer and will reproduce easily in your tank. Not harmful at all.