Why does my new cat bat and hiss at my chill cat?

we got the new small white cat over a month ago, we’ve been introducing them since we got her, been feeding together, having them sniff each other through the door, swapped toys and blankets, we sometimes let them out in the living room together and they’ll get along for a bit until the new kitty will randomly come up to our tabby and hiss and bat him. He doesn’t hit her back or hiss back. He just walks away. What does this mean and what should we do??

Help pls