As BENGA mentioned...
We need a complete posting of your latest water parameters. Be sure to include your temp and salinity in addition to all of your test numbers. In addition, it would be helpful to provide us with a list of equipment. What kind of lighting do you have on the tank? What about filtration? Do you use a sump? Do you have a protein skimmer?
The nice thing about Nems, if they are unhappy due to light and water FLOW, they typically will move to an area of tank which provides them with the light and flow they need.
Generally speaking, Nems can be difficult to keep. Tanks need to be well matured before you can successfully keep them.
I'm going to go out on a limb and guess that you have a new tank?