Is it possible that IPv6 multicast address(i.e address from
ff00::/8 address block) is shown in the "IPv6 Address" column of the "show ipv6 neighbors" command in Junos?
Multicast is not assigned to any particular host, so it should not be an IPv6 neighbor. Multicast addresses are destination addresses, not source addresses. Hosts subscribe to listen to traffic sent to a multicast group, but they do not send packets with source multicast addresses.