crystal_md is one of the Genesis variants of the default tilesets. Most likely, Nepster does not have a copy of this tileset, and does not have graphic set auto-downloading enabled (as this is an autodownloadable one). If this guess is correct, immediate solution is to either turn that option on, or to manually download crystal_md (either via browser or using the Download menu in the editor or Flexi) and place it in the "styles" folder.
Longer-term solution, GigaLem should include in his NXPs any graphic sets that aren't built into NL itself, even if they're auto-downloadable. This means any graphic set that isn't from L1, ONML, L2, L3 or the Lemmings Plus series, except for Xmas, Sega and Horror. Okay, this doesn't make sense, upon inspecting the NXP's contents there is a copy of crystal_md in there, so the presence or lack thereof of crystal_md in the styles folder should not matter.
There have also been reports that this pack does not load properly on (privately released) experimentals for new NL versions. While I've yet to properly investigate (I'm about to do so now), I suspect that this is due to GigaLem preferring to manually overwrite the default Lemming sprites with the Xmas ones, rather than simply selecting in Flexi Toolkit to use Xmas sprites on all levels and updating to the NeoLemmix version that fixes the bug where this option did not work. Second possibility is the use of actual custom spritesets that are lacking some animations, most likely the Fencer.