Unfortunately effects like CC Particle World cannot be masked directly, but you can use an Alpha Matte with a mask. I would have thought using a pre-comp should have worked, but I haven't tried that myself. Its possible Collapse Transformations is activated.
Is this even a mask and not a shape layer on top of the particles? The blue color is suspicious, as rthe default mask color is pale red. Everything else will require better info like an actual full screenshot of the timeline and comp with the settings revealed. Cropped images of the comp only are pretty much useless for diagnosing such issues. And of course other things liek system info and so on may also be relevant.