How secure is mongodb with elastic beanstalk and EC2
If eavesdropping of traffic intra-cloud is a potential issue for you, you really should implement the connection over SSL. 
You should also consider running all of your resources in a VPC (virtual private cloud). This allows you to isolate all of your EC2 resources in a private network that is isolated from the rest of the traffic. It also enables you to control where the points of connection to the external world are and how they behave.