Māhū as well as the other terms in the Culture-exclusive Genders folder are genders that are specific to certain cultures and should not be used by people not of that culture. If you're not sure you're able to use this, feel free to send an ask on ask-pride-color-scheme.tumblr.com, I have followers that could help you out!
From the submission [Link]:
Māhū "is a culture gender from Hawai'i, and it’s basically a sort of umbrella term for trans and nb. Trans women, trans men, bigender, and other nb/trans identities would be considered Māhū! They served spiritual roles, much like hijra and two-spirit, and were well respected by their communities.
Unfortunately, this is one of those genders that has been demonized by colonizers (see here for a concerning case, warning for police brutality), though there is certainly a push in recent times to return recognition and respect to all Māhū! This and this are interviews/documentary with Māhū.
For the flag, the plant is meant to be what is known in Native Hawaiian as pamakani mahu (mahu, Māhū!). The colors from the background come from the Kanaka Maoli, which is a flag that was made to represent Native Hawaiians (Important note! Some people spread misinformation that the Kanaka Maoli was the original Hawaiian flag before being colonized, but back then we didn’t have flags. The Kanaka Maoli is a modern creation (see here for more history on the flag), but still an important creation due to its representation of our native culture and heritage)."