What version are you running?
Interestingly if you look at the 3rd line of the output it seems to indicate that the CPU is actually just under 50% idle.
Does re-booting it make any difference?
If it is a recurring problem, then I'd recommend raising a service request with support.
I have a number of customers with appliances running 8.2.1 and I'm not aware of any of them reporting this kind of CPU usage from the ACL daemon.