Nasty little buggers, they will sting your corals and infest an entire tank if u let them mulitply. Inject with lemon juice, "joe's juice", or hot water to get rid of them.
Peppermint shrimp will also eat them.
Hypodermic needles are becoming hard to come by. If you know someone that uses insulin for diabetes just ask them for one. Some Walmart pharmacies will sell them to you but require that you buy them in a case of 100.