fear not fellow reefer......it doesnt sound like mantis shrimp rather copepods!!! which are very beneficial to your reef ecosystem. they reproduce rather quickly and are great food for smaller fish and faster moving inverts that are able to catch them....trust me, when i saw my first copepod i flipped out and thought of the worst for my tank but quickly realized the benefits they have for the aforementioned ecosystem.

happy reefing, steve