Identifiers of custom finalizers consist of a domain name, a forward slash and the name of the finalizer.

-- https://kubernetes.io/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definitions/#finalizers

For example:
"finalizer.acme.cert-manager.io" => "acme.cert-manager.io/finalizer"

This error triggers this output:

W0910 20:07:22.491920 1 warnings.go:70] metadata.finalizers: "finalizer.acme.cert-manager.io": prefer a domain-qualified finalizer name to avoid accidental conflicts with other finalizer writers