I think it's not a MAC address and it's IPv6 address.
MS switches can alert an administrator if a device is detected that appears to be using Network Address Translation (NAT). NAT on the LAN may indicate the use of a rogue AP or router, but also may be used for legitimate purposes with technologies such as containers, or tracking prevention.
https://documentation.meraki.com/MS/Monitoring_and_Reporting/NAT_Detection_on_MS_Switches
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
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