how do I clean my iMac of malware

The free Malwarebytes may take care of it...

https://www.malwarebytes.com/mac/

See if you have unknown Profiles.

To remove a configuration profile in macOS:

1. From the Apple menu, select System Preferences....
2. From the View menu in System Preferences, select Profiles.Note:
3. Profiles won't be visible until you have at least one profile installed.
4. Select the profile you want to remove, and then press the - (minus) button. Click Remove to remove the profile.

See also:

Effective defenses against malware and ot… - Apple Community