A slow cooker slowly heats up and then cooks the food and to cook the food the slow cooker requires it to get hot enough.
So if you notice the outside of the slow cooker getting hot to the touch then it's normal and nothing to worry about.
Anything that produces heat will naturally get hot on both the inside and outside and there's nothing wrong with it.
Unless you smell a very hot burning smell then the slow cooker getting hot on the outside is nothing to be concerned about.
My slow cooker has always gotten hot on the outside when it's cooking and I've left it on overnight for years and even when I've been away at work and my house has never burnt down.
Just make sure you don't put anything flammable near the slow cooker and you'll be just fine.