Hi there @alanfisher,

This looks like a crypto miner. What I could suggest is running the following command to get more information about the process:

ps aux | grep -i nJnK4xPK

Or change the nJnK4xPK part with the command that you currently see in htop.

This should show you the exact command that was used to start the process. Then you could kill the process but also delete the files associated with that process.

Then depending on where the malicious files are stored at you could figure out how they got there. For example, if you have a WordPress website and you find the files in your WordPress theme folder, then it most likely means that the theme was compromised. If you find the files in the /tmp folder then it is likely that some of your plugins were compromised and were used to upload the files there.

Depending on the software stack that you have you could then install the necessary updates and secure your website. If you are using WordPress you could follow the steps on how to do that here:

https://www.digitalocean.com/community/questions/how-to-secure-wordpress-without-a-security-plugin

Let me know how it goes!
Regards,
Bobby