You use unsupported Elastic db for graylog. Graylog version 3.3 supports only 5.6.x, 6.8.x of Elastic Search.
https://docs.graylog.org/en/3.3/pages/configuration/elasticsearch.html#elasticsearch-versions
Elastic 5.4 doesn’t have support for diskTotal parameter:
https://www.elastic.co/guide/en/elasticsearch/reference/5.4/cat-nodes.html
Solution: Please upgrade at least to version 5.6:
https://www.elastic.co/guide/en/elasticsearch/reference/5.6/cat-nodes.html