I would assume that you have Arc-enabled your SQL Server? Or for that matter, you have Windows for Arc. If you do that, Azure Arc will automatically enrol the SQL Server instances on the machine in Arc.
To find out in the portal, go to the Services page and search for Azure Arc. Once on that page, select All Arc Resources in the menu to the left (it's the second-highest item). This will show you a list of servers. All server names will typically appear twice, once for Windows and one for SQL Server. You see the type of service to the right in the list.
On the menu to the left under Security you find Microsoft Defender for Cloud. I would guess this is where you need to go turn it off. (I have not tried this service myself.)