It's a "Modified Wren Phoenix crest."

Different colored versions of it appear on some Rebel flight helmets. While it doesn't seem to be associated with any particular squadron, it most frequently appears on Blue Squadron helmets.

The Rogue One Ultimate Visual Dictionary writes that

The personalized iconography worn by pilots on their helmets occasionally identifies their unit origins, homeworlds, or beliefs.

It also gives examples of various pilots who wore the symbol.

[Photos of varying quality incoming.]

Blue Five:

Blue Four:

Gold Six:

Blue Nine:

Blue Eight

Blue Eleven

It was also available as an insignia option in the now-defunct official "Rogue One Helmet Creator."

Sabine Wren is a character on Star Wars Rebels who frequently paints this symbol, which the symbol in question is apparently based on.