Has he always done this? Do you know Mickey's past history - did he live with other cats who perhaps kept stealing his prey? Does he do this with non-lifelike toys (socks, strings)?
I had a young cat who did the growling thing whenever he "caught" a lifelike toy, up until he was nearly three years old. He would growl, but not hiss, and I never tried to physically take his "prey" away. Eventually he stopped the growling (but I never did try to take his prey away, when he had it in his mouth or had his paw on it.)
Mickey's behavior sounds like he may have a history of someone (or another animal) repeatedly infringing on his territory and taking his prey/toy. I remember a neighbor's dog acting that way when the son would get too close when the poor dog was trying to eat, because the son had a history of taking the dog's food away and playing "keepaway." (This was not a nice-to-pets kid.)
It's my guess - only my guess, based on conjecture from seeing other animals who act this way - that Mickey may take years to outgrow this behavior, or to learn that you (or someone else) isn't going to take his prey.
Whenever Mickey grabs one of his toys and starts growling, lower your hands and tell him what a good hunter he is. Just keep that up until he stops growling - then pet him and praise him again. Do not try to take the toy away. See if he's still in the mood to play by getting another toy and lightly tossing it from hand to hand. If he drops Toy A and looks ready to play, throw Toy B. Repeat until he's done playing, always praising him but never making a move to take whatever toy(s) he has in his mouth or under his body. (This may require buying a few multi-packs of toys.) Eventually, he'll realize that you are truly playing with him, and aren't going to take his toy. He may get to the point of bringing one of his toys to you and dropping it so that you'll throw it again (all of my cats have done this, including the feral-returned-two-times-before-I-adopted-her rescue kitty.) But he has to learn to trust you, and that can take some time for any cat, let alone a higher-strung cat like Mickey.