Two of my cats have done this to me. I checked with my vet - she calls it "displaced aggression". Your furball wants to defend his territory and since he can't get to that other cat, takes it out on the closest animal/person. My vet suggested not touching my kitty or otherwise trying to distract her because that can make the situation worse. They're so focussed on the interloper that they turn on you. Just leaving my cat alone and/or leaving the room has worked best for me.
Good luck!