A
It's a website notification. See this support article for instructions on how to deal with it.
B
"CleanMyMac" is a scam and a common cause of instability and poor performance. Depending on what version you have, the developer's instructions may not completely remove it. Follow those instructions, then do as below.
Back up all data before proceeding.
Triple-click anywhere in the line below on this page to select it:
/Library/LaunchDaemons/com.macpaw.CleanMyMac2.Agent.plist
Right-click or control-click the highlighted line and select
Services ▹ Reveal in Finder (or just Reveal)
from the contextual menu.* A folder may open with an item selected. If it does, move the selected item to the Trash. You may be prompted for your administrator login password.
Repeat with this line:
/Library/PrivilegedHelperTools/com.macpaw.CleanMyMac2.Agent
Restart the computer and empty the Trash.
You may also have to remove one or more of these items in the same way:
~/Library/LaunchAgents/com.macpaw.CleanMyMac.helperTool.plist
~/Library/LaunchAgents/com.macpaw.CleanMyMac.volumeWatcher.plist
~/Library/LaunchAgents/com.macpaw.CleanMyMac3.Scheduler.plist
Never again install "CleanMyMac" or anything like it.
*If you don't see the contextual menu item, copy the selected text to the Clipboard by pressing the key combination command-C. In the Finder, select
Go ▹ Go to Folder...
from the menu bar and paste into the box that opens by pressing command-V. You won't see what you pasted because a line break is included. Press return.
C
You should never use any kind of "anti-virus" or "anti-malware" software on a Mac.
