I'm developing gRPC microservices that running in Kubernetes cluster with insecure connection.
Should we apply SSL/TLS connection between service in Kubernetes?
But anyway, this might be overkill to apply TLS for internal connection in k8s cluster.
This solely depends on what level of security you want to have. If you decide to use mTLS between services, this is typically implemented using a "service mesh", e.g. Istio.