You are using Slack and wonder if the Slack Chat is monitored as an Instant Messenger.
Currently Slack Chat is not monitored as an Instant Messenger by the DLP Endpoint or Network Monitors. You are able to monitor clipboard activity when copying from Slack however.
For more information about supported Instant Messaging (IM) applications please refer to your Administrator's Guide.
There is an open Enhancement Request to add Slack chat into application monitoring: PM-3040.