Q: Help - how to get rid of this: "Please consider cleaning your Mac from junk."

Fixes for Adware and Pop-ups

1. Malwarebytes
2. DetectX 2.11
3. Remove adware that displays pop-up ads and graphics on your Mac
4. Stop pop-up ads and adware in Safari - Apple Support

[Please ignore remarks such as "Don't use any kind of "anti-virus" or "anti-malware" product on a Mac." Such remonstrations are an exaggeration. They may be needed in some situations, but need not be installed or used for all circumstances. Furthermore, adware removal programs make the job of removal much easier. They require no permanent installation to remove adware or other types of malware. They do no damage, and they don't make you more vulnerable to attack.]

Fixing Safari from Pop-ups

[The following comes from user stevejobsfan0123. I have made minor changes to adapt to this presentation.]

Fix Some Browser Pop-ups That Take Over Safari

Common pop-ups include a message saying the government has seized your computer and you must pay to have it released (often called "Moneypak"), or a phoney message saying that your computer has been infected, and you need to call a tech support number (sometimes claiming to be Apple) to get it resolved. First, understand that these pop-ups are not caused by a virus and your computer has not been affected. This "hijack" is limited to your web browser. Also, understand that these messages are scams, so do not pay any money, call the listed number, or provide any personal information. This article will outline the solution to dismiss the pop-up.

Quit Safari

Usually, these pop-ups will not go away by either clicking "OK" or "Cancel." Furthermore, several menus in the menu bar may become disabled and show in gray, including the option to quit Safari. You will likely have to force quit Safari. To do this, press Command + Option + Esc, select Safari, and press Force Quit.

Relaunch Safari

If you relaunch Safari, the page will reopen. To prevent this from happening, hold down the 'Shift' key while opening Safari. This will prevent windows from the last time Safari was running from reopening.

This will not work in all cases. The shift key must be held at the right time, and in some cases, even if done correctly, the window reappears. In these circumstances, after force quitting Safari, turn off Wi-Fi or disconnect Ethernet, depending on how you connect to the Internet. Then relaunch Safari normally. It will try to reload the malicious web page, but without a connection, it won't be able to. Navigate away from that page by entering a different URL, i.e. www.apple.com, and trying to load it. Now you can reconnect to the Internet, and the page you entered will appear rather than the malicious one.

Safari – Popup takes over

Popup remover – Scam Zapper