Is My ISP an IPv6 Client on My Network?

My ISP is Spectrum. I noticed my gateway (ER-X running OpenWrt 19.07.4) has an active lease ending with "". Yikes! Does this mean my ISP is somehow on my network as a client able to access who knows what?

No. It's probably just Spectrum has registered a reverse DNS for the IPv6 subnet being used. So when OpenWRT tries to find a hostname that's what it finds when it queries DNS.

In short, it's nothing to worry about.