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?
It is running 8.2.1P03. There are 2 processors, one is busy, the other is idle. Rebooting will fix it, but it will happen again in a couple weeks.
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.