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Hi there and welcome
This appears to be a fake malware page. Its not an actual infection but a fake page to try to trick you to ring the number and pay money for a non existent issue.
They aim these more towards windows computers, hence the windows error.
your not going to lose data, but From your Safari menu bar click Safari > Preferences then select the Advanced tab.
Select: Show Develop menu in menu bar
Now from the menu bar click Develop > Empty Caches
Close Safari, open it and tell me if it now operates correctly please