Hmm, ok, this is strange. At some point in the past has this node changed role, e.g. by changing node.data: true to node.data: false?
Since the cluster health reports as green I think the simplest thing to do is wipe the data folder on this node and start it afresh. Be double-sure that the cluster really is green first (i.e. that you're not looking at a different cluster's health).