I joined a ring, then left it

now my local ring is just showing my local cassndra node, like intended

but on the cassandra -f process, It keeps sending every 5 seconds

WARN 16:01:44,042 ClusterName mismatch from /123.123.123.123 big cluster!=Test Cluster

where big cluster is the cluster name used when in the previous ring


Why do I keep receiving this alerts? is it because 123.123.123.123 still tries to be in the ring?
Should I run nodetool decommission (token that had my node) on that node? to stop this alert?

thx for explanations