If its new to the tank it will probably move until it finds its happy place. Try and wait for that to happen before moving powerheads or anything as that will keep it moving. nems will also move when they are not happy, if the right conditions aren't being met (not enough light/flow/dirty water) be careful if you have a reef as they can sting corals along their way... Hopefully of its new it will settle soon (hopefully not at the back of your tank!) also, don't move it yourself as it will likely move itself back.