According to Cisco.
NAT66—Translates IPv6 packets to a different IPv6 address. We recommend using static NAT. Although you can use dynamic NAT or PAT, IPv6 addresses are in such large supply, you do not have to use dynamic NAT.
Firepower Management Center Configuration Guide, Version 7.0
however, this does mean that for host to host NAT you must use auto-nat to create a usable address.
once this is setup you should be able to start seeing hits from the CLI
show nat
Auto NAT Policies (Section 2)
1 (dmz) to (outside) source static WebServerv6 ipv6-att-slaac
translate_hits = 586, untranslate_hits = 145