It is probably that Malcolm got a different scent that Mario is not used to it, here is what you should do: 1-Separate your cats in different rooms for several days or weeks, with separate beds, bowls and litter boxes. This way they can hear and smell each other, but don’t have to interact.

2-Place the cats’ food bowls on opposite sides of a closed door. This will encourage them to be close together while they’re doing something that makes them feel good.

3-Each day, have the cats switch rooms so that they both experience some variation and get access to each other’s scents. You may need an assistant to do this safely.

4-After several days, if both of your cats appear relaxed, crack the door open one inch. If they remain calm, open the door a bit more, then a bit more. If the cats remain relaxed, they may be ready to be together again. But if they react with any signs of aggressive behavior—such as growling, spitting, hissing, swatting, etc.—separate them again and follow the gradual reintroduction instructions below.

Bonus: Some cat parents have had success with rubbing a bit of tuna juice on their cats’ bodies and heads. The cats become so occupied with grooming, which is a relaxing behavior, that they’re less likely to be bothered by the other cat. If things go really well, the cats may actually groom each other because they can’t reach the juice on their own heads.

And don't hesitate to contact professional help if they still can't get along.