AWS Inspector "High Risk" : port open to the internet with IP whitelisting
If it is FTP (and not FTPS) then yes, it is a high severity risk. You are sending unencrypted traffic over the open internet. Even if you are whitelisting specific IP addresses, the traffic is still unencrypted between those specific instances.
In addition, FTP is a very old (created in 1971) and non-secure protocol. It is not meant to be a secure method of transferring files. I recommend you research some of the security concerns with FTP and switch to either FTPS or more preferably, use a more native AWS approach for transferring files between instances. If your organization is under any regulation (FEDRAMP, HIPPA, etc.), using FTP across the open internet will not meet the compliance requirements.