Is it safe to use uNmINeD while Minecraft is running?
Java Edition: most likely yes, as no one has ever reported a problem in the ten-year history of uNmINeD.
Bedrock Edition: maybe yes, but uNmINeD is likely to crash when world files change.
It’s always a good idea to close Minecraft before using an external tool, and make regular backups of your worlds to prevent data loss.
Is uNmINeD a malware/virus?
Absolutely not. Some cautious antivirus tools may consider it dangerous because it is a less common software that relatively few people download, and it is not signed by a certificate because software signing certificates are expensive.
Be careful not to download uNmINeD from anywhere other than this site (unmined.net). There are plenty of malicious sites that offer fake minecraft tools for download.