I haven't heard about DC RUM monitoring ElasticSearch. External communications that use HTTP/HTTPS can for sure be monitored with the Web decode, most efficiently using rules defined to extract parameters specific to your interest.
For sure we don't have decodes specific to the ElasticSearch internal protocols. Of course DC RUM's universal decode can be used to build a decode for pretty any TCP-based protocol, but the effort might be not worth the effect - there are alternatives that may bring the result faster in a more actionable way. Have a look at https://answers.dynatrace.com/spaces/148/uem-open... - API approach to monitoring it with AppMon looks like a ready-to-use alternative.