So the prostate is an internal organ, and not one you can see. I'm not sure where your bruise is, but you wouldn't see it on your prostate.
Is the bruise somewhere that your hand moves on a lot when you masturbate? That could cause bruising. Maybe you squeezed too hard, or got a little rough with yourself.
Is there any pain, tenderness or soreness? Do you think this patch is a bruise or does it feel like rough skin?
You don't say how long it's been there, but if it's been more than a few days, you should get it checked by your doctor.