IPv6 --> Neighbors
https://wiki.mikrotik.com/wiki/Manual:IPv6/Neighbors
The hosts are auto configuring the IPv6 address via SLACC. The address used will change often if the host has Privacy Extensions on (which is most likely the case).
Note that the DHCPv6 server in the MT currently only supports Prefix Delegation, not host address requests .... so, nothing to see there.