A wet dream or prone masturbation ?

I don't know how much this is setting you back. There is the psychological issue of quitting (your mind), and then there's the neurological issue of your brain being accustomed to dopamine from pmo. I don't know if these ejaculations are continuing to have the same effect neurologically when you're asleep as when you're awake though.
I agree that this shouldn't set your counter back, especially if you are quitting for moral or religious reasons. If you don't do it at least partly of your own free will, then you shouldn't feel any guilt over it.
As for stopping it, you could try to consciously condition your feelings about it to view it as bad/undesirable. That might allow you to make it stop even when you're semiconscious.
Also checkout this related thread. http://www.nofap.com/forum/index.php?threads/wet-dreams-a-dealbreaker.36970/#post-233862

 

Many of my relapses are a result of prone masturbation. I'm used to it. But my wet dreams were usually when I lied on my back at night!

I suggest you clear your head from all fantasies and sensual imagery. Before you sleep, do a 10 minutes meditation. Also, most nights I sleep, I put a wallet on my penis underneath my clothes or a plastic wire. That prevents your penis from having contact with soft things and rub against it. It works. We're the unlucky fapstronauts we must have other precautions!