You can assign it more memory through the "Configuration" page.
Click on Cloudera Management Service -> Configuration -> Select "Event Server" on the filters on the left.
Add "heap" to the search box at the top of the filters. You should see "Java Heap Size of Event Server in bytes" as a configuration option.
Try increasing it to 512MB or 1GB.
It would be nice to get an idea of the whole log, to see what its doing that led it to run out of memory.