What is this skin disease? #catstr #cat 
Discussion
Poor baby
It seems like a type of infection caused by bites from other cats. A pustule forms with pus and then bursts. It requires treatment with local and oral antibiotics.
If there are no injuries, look to what you're feeding the cat.
What kind of food can cause that ? I give many type of food, sometimes leftovers but never ultra processed human food.
When i don't have leftovers i give her paté now maybe that's that, she's not use to processed cat food
As the vet, they’ll take a sample and confirm what it is if they can’t tell just by looking at it.
My guess is like you said, a skin disease. Either way, completely changing their diet would be worth doing, ask the vet.