The main law you will probably need to be concerned with is trespassing. I assume that you will be asking the mall for permission?
If you have anyone walking around in costume anywhere, go down to the police station first (don't call) and tell them what you are doing so it doesn't come as a surprise. A surprise for them will work out badly for you.
If you get everything worked out, carefully think about the type of scares you are going to do. People going to your house expect to be scared. The person that throws seven dollars worth of ice cream when you sneak up on them at the mall won't be expecting it and will want to be reimbursed.
I have done this before and set it up with someone dragging along a shackled creature. This gets just as much attention and the person doing the leading can pass out flyers for your haunt (get permission before handing out the flyers since many of them will end up on the ground/floor).
Think very carefully about having someone sneaking around scaring people. Besides being annoying to some, you need to get the word out about your event. It might be fun for the person doing the scaring but if you alienate potential customers and have no effective way of telling them about your attraction it could be a waste of time.