Cat is suddenly hissing at a previously friendly cat?

Hello everyone,

I usually take my cat (2 years old, castrated male) outside on a leash every afternoon, I have trained him to accept the harness since he was a kitten and he greatly enjoys his walks.

Now, last year a new cat appeared. I call him Yapper (he meows a lot and I don't know his real name). I don't know if he belongs to someone, he looks well fed and cared for but isn't castrated and doesn't have a collar. Yapper has a habit of following us around during our walks. If he sees us from far away he comes running to meet us, it's very cute. Yapper will sometimes try to play with my cat but he usually ignores him.

The current issue, my cat has started hissing at him. He has never hissed at another cat before, usually my cat is the one getting hissed at by others (not Yapper). I don't understand, Yapper hasn't done anything except getting close, he tries to meow at my cat softly. I thought maybe my cat was just annoyed that day, but today the same happened.

My cat saw Yapper from afar and went up to him, I thought he was saying hi like usual but as soon as Yapper meowed softly, my cat started hissing and getting away from him.

I don't get it? Yapper looks so sad about it and my cat seems stressed and is cutting his walks short because of it.

Anyone knows what this could be????