Here's a picture of one: If you have one, you can either keep it by itself in a different tank or get rid of it, because mantis shrimp are quick and have been known to kill fish.
Let's start with the basics. What stock is in the tank? List off the contents.
There are more possibilities than mantis shrimp - pistol shrimp for one, even some clownfish can pop very loudly with their mouths.
Are the clicking rapid or just every now and then? If they are rapid then it will most likley be a mantis if they are every now and then it may be a pistal shrimp.