Could mean anything, if it is on more than one helmet, it might narrow down possibilities.
It could be a symbol for Military:
- a division of the Alliance
- Infantry
- special forces
- symbol of authority
It could have personal meaning
- Planetary symbol
- Sports team
- Star Wars universe version of a pot leaf
- A family crest
- Initial of a name
- religious symbol
Or totally random:
- Price tag left on it from the army/navy surplus store
- This side forward
- Allergic to latex
- Star Wars universe emoji similar to poop
See... anything.