155.xxx.xxx.x.x is an IPv4 address.
2601:7:6bc0:2a:484c:cf96:6f88:65bd is an IPv6 address.
It's possible for a computer to obtain and use both IPv4 and IPv6 addresses simultaneously.
mDNSResponder is used by Bonjour, the "automatic" service-finding protocol by Apple. Probably, your computer is periodically sending out "Bonjour" packets and listening for responses from other Bonjour-enabled computers.
It's nothing to be worried about.