pop up malware problem: "This Mac can't connect to iCoud because of a problem with "myemailaddress." Open iCloud preferences to fix this problem."

You can use Malwarebytes for Mac — Mac Antivirus Replacement | Malwarebytes

And scan the Mac or can install etrecheck

And find out unidentified apps installed in the Mac .

See these articles Safari Privacy preferences on Mac - Apple Support

Clear your browsing history in Safari on Mac - Apple Support

Prevent websites from tracking you in Safari on Mac - Apple Support

Safari Websites preferences on Mac - Apple Support

Customize settings per website in Safari on Mac - Apple Support

Did you open the System/iCloud preference pane and see if there were any item in there that needed tending to? Check to make sure you're logged in to iCloud in the preference pane.

Open the preferences by going to the Applications/Utilities folder and launching System Preferences. That way you keep it local if it is malware.

No, it's not. A popup is the same as any other web popup. This one just happens an obnoxious and lame attempt to get you to give crooks your personal information.

Malware is software installed on your device. That would be a virus (of which none exist on any Apple device), a Trojan (which is software YOU knowingly, or unknowingly install), or a worm. All known malware on Macs are Trojans. That includes keyloggers, back doors, adware, and anything else that requires getting the user to install it. It can't get there by itself.

There is no malware for iOS. None. Not unless you've jailbroken your device. Then all bets are off.

A simple web JavaScript popup is not malware. Nothing was installed on your device. JavaScript is just how part of the web works. This is what you're seeing. A JavaScript popup, or redirect to another site designed to look like a popup. If the URL on top hasn't changed, it's a popup. If it does change, you've been redirected. Either way, it's still just JavaScript. Not malware.

The email or text message is a scam. Apple would never ask you for your SSN or credit card info. Do not respond to the message in any way. Simply delete it.

And no, it's not malware. It's just a garbage email from a scammer. No malware of any kind is necessary to do anything that simple.

See this Identify legitimate emails from the App Store or iTunes Store - Apple Support

We know. My wife kept getting a pop up warning when booting regarding issues with iCloud login. It had to do with email login in System preferences. The OP wasn't specific that the popup came from his browser which we now know.