I found a solution to remove the dust from the protector :
Take 2 post-its,
Put them back to back, shifted 5mm one from the other, their sticky parts also back to back.
Fold this resting 5mm wide band of the sticky border of one post-it, and stick it to the sticky part of the other post-it.
You should end up with a double sided post-it. Like a double sided tape, but with post-its.
Then pull the protector up, slide the post-it between the screen and the protector, press, and this should remove the dust
My protector was messed up with a lot of dusts, and I could totally clean it. It takes time though.
Don't hesitate if it's not clear (this description was hard to write
)