To a Predator, it is a mask.
To a prop maker who makes predator masks, they need to differentiate between a predator mask (giving a human the look of a predator face) and the mask worn by Predators so they call it a bio helmet.
At least, that is how I see it.
Since the movies and novels are dealing with Predator characters, they only have one mask, and it is called a mask, not a bio helm. It is only people who want to mimic a predator that has both.