I would say to get some coil cleaner and spray it,but you probably wouldnt wont to do that since its on your tank.I dont know that it would actually hurt anything as long as it didnt get into the water,but I wouldnt risk it.
SO
Take a stiff bristle tooth brush and the vacuum.And like Bj said,be careful not to bend the fins.You'll probably see a big difference in it after its cleaned.A chiller is basically an air conditioner for the tank and works the same as the unit that keeps you cool.