What is this? Pop up in Safari with red warning sign saying something about an ip address?

[Wild eep]

GeneL wrote:

Could it just be a transient glitch and nothing to worry about?

Yes.

If it is working for you now then any effort to find out what happened is a waste of time.
If it happens frequently then troubleshooting is in order.

Some routers do get confused for a moment and clears it up.

It is what it is.