Costumed attendees must wear masks under helmets if the helmet is likely to be removed while inside the convention. If you can wear your helmet (or other face covering) without removing it while inside the convention space, you don’t have to wear a mask underneath it.
Costume contest contestants must wear Covid masks until they get on stage. They may then remove them to have the chance to show their costume, if they wish. Contestants will need to put them back on once they finish and walk off stage.