Hey micki193,
If you haven't already, you could see if restarting your iMac might stop that message you're seeing by going to the Apple menu > Restart.
Another step to possible help stop this would be to start in safe mode, as described below. Safe mode only allows certain software to load, which might help stop this downloading message:
"How to use safe mode
Determine whether you're using a Mac with Apple silicon, then follow the appropriate steps:
Apple silicon
- Shut down your Mac.
- Turn on your Mac and continue to press and hold the power button until you see the startup options window.
- Select your startup disk, then press and hold the Shift key while clicking “Continue in Safe Mode.”
- Log in to your Mac. You might be asked to log in again.
Intel processor
- Turn on or restart your Mac, then immediately press and hold the Shift key as your Mac starts up.
- Release the key when you see the login window, then log in to your Mac.
- You might be asked to log in again. On either the first or second login window, you should see ”Safe Boot” in the upper-right corner of the window."
Those steps are from this article: How to use safe mode on your Mac
If the issue continues, please answer the questions below as it'll help us understand what's going on better and possible next steps.
- Has there been any recent changes to your iMac before you noticed this? This could include a new app, settings change, so on.
- Have you recently tried to download content in an app that might be causing this message?
Take care.