Apple Care Scam - How to respond?

It can't install anything on your computer just by being a popup. You would have to click on something to initiate a download. You should be fine.

Yes, it's a scam.

Close and reopen Safari.

Learn how to identify and avoid fraudulent "phishing" email, fake ‘virus alerts’ and other scams:

https://support.apple.com/en-us/ht204759

How to report phishing emails to Apple:

These email addresses are monitored by Apple, but you might not receive a reply to your report.

• If you receive what you believe to be a phishing email that's designed to look like it’s from Apple, please send it to reportphishing@apple.com.
• To report spam or other suspicious emails that you receive in your iCloud.com, me.com, or mac.com Inbox, please send them to abuse@icloud.com.
• To report spam or other suspicious messages that you receive through iMessage, tap Report Junk under the message, or take a screenshot on your iPhone, iPad, iPod touch, or Mac and send it to imessage.spam@apple.com.
• If you receive a suspicious message about your purchase activity in the iTunes Store, App Store, or iBooks Store, contact iTunes Support.