My Fierce Flamberge does pure piercing, so it should be neutral on Gremlins, and it is. However, menders (all three tiers) display the small gray damage numbers when getting hit by it.
I don't have any other pure piercing weapons to test this with atm, so it could also just be this sword.
It's a known bug. Piercing damage shows up as grey on Menders when a weapon is doing the same damage to a Thwaker, Demo or Scorcher.