Question about cats.

Friday night i was coming home and a white cat (which I have seen at least 3 times) started following and meowing, i came closer and heard it was purring so petted it, a few minuets later i picked it up and put it in my house and gave it a bath, and my other cat starts hissing anytime the other cat comes near, why is he doing this, and how can I prevent this

NOTE-the outside cat is white female
the inside one is black and male

Question

Question about cats.

Friday night i was coming home and a white cat (which I have seen at least 3 times) started following and meowing, i came closer and heard it was purring so petted it, a few minuets later i picked it up and put it in my house and gave it a bath, and my other cat starts hissing anytime the other cat comes near, why is he doing this, and how can I prevent this

NOTE-the outside cat is  white female 
the inside one is black and male