EC2/UnusualDNSResolve- Guard Duty

Hi, see https://docs.aws.amazon.com/guardduty/latest/ug/guardduty_finding-types-ec2.html#defenseevasion-ec2-unusualdnsresolver for details

An Amazon EC2 instance is communicating with an unusual public DNS resolver.
Default severity: Medium

Data source: VPC flow logs

This finding informs you that the listed Amazon EC2 instance in your AWS environment
 is behaving in a way that deviates from the baseline behavior. This EC2 instance 
has no recent history of communicating with this public DNS resolver. The Unusual 
field in the finding details panel in the GuardDuty console can provide information 
about the queried DNS resolver.

Remediation recommendations:

If this activity is unexpected, your instance may be compromised. 
For more information, see Remediating a compromised EC2 instance.

So, basically, it says that your instance has started to talk with a new DNS server (it never queried it before). This unusual resolver will be listed on the Gard Duty panel. Check if it's a safe DNS resolver or not. If not, you will have to prevent your instance from talking to it.

8.8.8.8 is Google DNS Servers. Nothing to worry about much on the security end unless you have a DNS Server that your instances should be using