Multisexuality is an umbrella term for any form of attraction to more then one gender. It is otherwise called 'm-spec' and can include pansexual, bisexual, polysexual, omnisexual people and more.
Flag
The flag was made official on February 13th, 2019 and the creator went by "synp".
Peace is represented by the purple bar at the top, and unity by white, light blue stands for free and the pink/magenta bar is for love and compassion.
Resources
https://www.deviantart.com/kinkflagsrus/art/Multisexual-Official-Pride-Flag-785458128