This keeps popping out and I can't make it stop!

The B8:09:8A:F1:DE:26 connection request, is coming from another Apple device like an iPhone, iPad or Watch.

If checking Ignore this device as suggested by IdrisSeabright doesn't help..?

You might be able to identify and ignore the device by going to Apple > System Settings... > Bluetooth.

If not, then open the Bluetooth Settings on your other Apple devices and set that device to ignore the Mac mini.

If all else fails boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

"This keeps popping out and I can't make it stop!"

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Is this message still here?

If so, try the following:

In post-Catalina, you can use Terminal to reset Bluetooth issues. See my first reply In my User Tip: Use "Reset the Bluetooth module" to Fix Constant Bluetooth Disconnections - User Tip. So, try the following...

1. Go to: the Desktop
2. Click: Go menu (at top of screen, by default)
3. Select: Utilities
4. Open: Terminal
5. Use this Terminal Command:

sudo pkill bluetoothd

Screenshot from my User Tip:

Have you selected "Ignore this device?"