https://ffxiv.gamerescape.com/w/images/1/18/Sylvie.png
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Hmmm, it's reminiscent of the Valentione Skirt (legs from the 2014 Valentine's Day event -- available on the Mog Station for some amount of $) dyed grey/black, however even when dyed, the darker stripes on those pants remain black, rather than this NPC's grey color. Also, I don't think the stripes go up that high on the skirt (they only go 1~2 stripes above the knee before cutting off).
I looked up the NPC on a database website, and it doesn't have her leg equipment listed properly (those definitely aren't Holy Rainbow Bottoms), so I suspect they're custom NPC-only legs for now.
Agree it looks like the Valentione Skirt. For reference:
https://na.finalfantasyxiv.com/lodes...fc755e/?patch=
https://xivstyle.com/glamour/event/valentiones1/