Presuming your mix is wet then try Java Moss which can grow in water or on any real wet surface.

Here is a link that might be useful: Java Moss

Any moss but pure sphagnum is a NO-NO for Nepenthes and most other CPs.

Only from what I've heard from other growers and not from personal experience (I don't grow Pings) Pings are the exception. Most pings including Mexican pings are higher in PH needing alkaline and (not) acidic soil/moss.

My Nepenthes mix is 1/1/1, sphagnum peat moss/course silica sand(#12 or larger)/orchid bark and topped with live sphagnum. You can sub the sand with perlite for a much lighter mix.