My cat has started hissing at me since another cat moved in

This could be what's called redirected aggression; he’s stressed and isn't too happy about another cat intruding on his territory, so he's reacting by being in a foul mood. Don't take his reactions personally. Just like Grumeau said, separate them if possible without disrupting the first one's routine. Give them each their own space, like a bedroom where one can leave their scent and have some peace and quiet. The second cat will just have to adapt. It took a few months to get to a point where they could live together peacefully without any hissing or growling when I moved back in with my mum with my cat, Minou. He understood perfectly well that he was in another cat's home, and my mum's cat knew how to set his boundaries. At home, I prefer to let them get on with it; even if one gets a swipe, it’s just their way of communicating. Even cats who have been best buds for years can butt heads from time to time. You still have to take their personalities into account, though, and don't let one get bullied, obviously! 😁 It’s all about a happy medium—not interfering too much in how they interact while still providing neutral spots where they can go for a rest. Generally, when a second cat arrives, you start by putting them in one room for a while. That way, they can start exploring without too much disruption. This room becomes their "home base", and if they get stressed, they can retreat there in peace. The first cat’s territory isn't too encroached upon, but they can still sense the presence of the newcomer.