What is this bottom piece?

What is the name of the bottom piece in this video, and where can I get it? Thanks.~

You can purchase that skirt from the Allied Seals/Hunt Billmaster NPC located in Ul'Dah inside the Immortal Flames GC area. Note that it is specific to:
--Being level 50
--Being a Disciple of Magic (it can be worn by all casters and all healers)
--Being in the Immortal Flames (it's Grand Company locked)

The item is called Flame Elite's Loincloth, and I believe it costs 1,000 Allied Seals.

Thanks, Hyomin!

You're welcome! ^^