It is represented by a blue circle with a skull in the middle. The skull has a plasma grenade on its forehead.
The Catch skull can be found on The Storm on normal or higher difficulty. This skull is one of the trickier skulls to obtain, since there are multiple factors included. If you get to the first AA Wraith don't destroy it, this will make the skull disappear. Instead, look at the 10 meter high cylinder that is to the right of the AA Wraith. Try to either get a Warthog there and jump from the Warthog on the cylinder or execute a well timed Grenade Jump. You could also get it with a friend and he could stand on top of you and you both jump at the same time, making his jump higher, and letting him reach the silo.